Published December 16, 2002
Non–T Cell Activation Linker (NTAL): A Transmembrane
Adaptor Protein Involved in Immunoreceptor Signaling
Tomáš Brdička,1 Martin Imrich,1 Pavla Angelisová,1 Naděžda Brdičková,1
Ondrej Horváth,1 Jiří Š pička,1 Ivan Hilgert,1 Petra Lusková,1 Petr Dráber,1
Petr Novák,2 Niklas Engels,3 Jürgen Wienands,3 Luca Simeoni,4
Jan Österreicher,5 Enrique Aguado,6 Marie Malissen,6 Burkhart Schraven,4
and Václav Hořejší1
1Institute
Abstract
A key molecule necessary for activation of T lymphocytes through their antigen-specific T cell
receptor (TCR) is the transmembrane adaptor protein LAT (linker for activation of T cells).
Upon TCR engagement, LAT becomes rapidly tyrosine phosphorylated and then serves as a
scaffold organizing a multicomponent complex that is indispensable for induction of further
downstream steps of the signaling cascade. Here we describe the identification and preliminary
characterization of a novel transmembrane adaptor protein that is structurally and evolutionarily related to LAT and is expressed in B lymphocytes, natural killer (NK) cells, monocytes,
and mast cells but not in resting T lymphocytes. This novel transmembrane adaptor protein,
termed NTAL (non–T cell activation linker) is the product of a previously identified WBSCR5
gene of so far unknown function. NTAL becomes rapidly tyrosine-phosphorylated upon crosslinking of the B cell receptor (BCR) or of high-affinity Fc- and Fc-receptors of myeloid
cells and then associates with the cytoplasmic signaling molecules Grb2, Sos1, Gab1, and c-Cbl.
NTAL expressed in the LAT-deficient T cell line J.CaM2.5 becomes tyrosine phosphorylated
and rescues activation of Erk1/2 and minimal transient elevation of cytoplasmic calcium level
upon TCR/CD3 cross-linking. Thus, NTAL appears to be a structural and possibly also functional homologue of LAT in non–T cells.
Key words: lipid rafts • membrane microdomains • antigen receptors • Fc gamma receptor •
Fc epsilon receptor
Introduction
Immunoreceptors (TCR, B cell receptor [BCR],* most
Fc-receptors) initiate, upon binding of their agonist ligands,
signaling pathways based on an inducible activation of Src-,
Syk-, and Tec-family protein tyrosine kinases (1–3). Eventually this leads to activation of further downstream signal-
Preliminary reports (abstracts, poster and oral communications) on some
aspects of this work were presented at the EMBO Workshop on Lymphocyte Antigen Receptor and Coreceptor Signalling, Siena, May 4–8,
2002 and at the ELSO2002 Meeting, Nice, June 29–July 3, 2002.
Address correspondence to Václav Ho ř ej š í, Institute of Molecular Genetics AS CR, Víde ň ská 1083, 142 20 Praha 4, Czech Republic. Phone:
420-2-41729908; Fax: 420-2-44472282; E-mail: horejsi@biomed.cas.cz;
or Burkhart Schraven, Institute for Immunology, Otto-von-Guericke-
University, Leipziger Strasse 26, 39120 Magdeburg, Germany. Phone:
0391-67-15800; Fax: 0391-67-15852; E-mail: burkhart.schraven@
medizin.uni-magdeburg.de
*Abbreviations used in this paper: BCR, B cell receptor; BMMC, bone
marrow mast cell; GEM, glycosphingolipid-enriched microdomain; LAT,
linker for activation of T cells; NTAL, non–T cell activation linker; PI3-K,
phosphatidylinositol 3-kinase; PLC, phospholipase C; PTK, protein
tyrosine kinase; P-Tyr, phosphotyrosine.
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J. Exp. Med. The Rockefeller University Press • 0022-1007/2002/12/1617/10 $5.00
Volume 196, Number 12, December 16, 2002 1617–1626
http://www.jem.org/cgi/doi/10.1084/jem.20021405
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of Molecular Genetics and 2Institute of Microbiology, Academy of Sciences of the Czech Republic,Vídeň ská
1083, 142 20 Prague 4, Czech Republic
3Department of Biochemistry and Molecular Immunology, University of Bielefeld, Universitätsstrasse 25, Bielefeld
D-33615, Germany
4Institute for Immunology, Otto-von-Guericke-University, Leipziger Strasse 26, 39120 Magdeburg, Germany
5Department of Radiobiology and Immunology, Purkyn ě Military Medical Academy, Tř ebeš ská 1575, 500 01
Hradec Králové, Czech Republic
6Centre d’Immunologie de Marseille-Luminy, INSERM-CNRS-Univ. Med., Parc Scientifique de Luminy, 13288
Marseille Cedex 9, France
Published December 16, 2002
Materials and Methods
Cells and Antibodies. Human T, B, NK cells, and monocytes
were obtained from buffy coats by Ficoll centrifugation and preparative cell sorting using a FACS Vantage™ flow cytometer
(Becton Dickinson) and PE-conjugated mAbs to CD3 and CD19
(Serotec), biotinylated anti-CD14 (Serotec), fluoresceinated
streptavidin (BD Biosciences), unlabeled CD56 mAb MEM-188
(product of the Prague laboratory), and fluorescein-conjugated
F(ab)2 fragments of goat anti–mouse Ig (Caltag). Mouse splenocytes (an unseparated washed suspension containing 40% B
cells) used in functional experiments with B cells were obtained
from C57/BL6 mice. Human blood monocytes used in the functional experiments were obtained by preparative cell sorting based
solely on their characteristic side and forward scatter properties
(i.e., without any antibody staining). Bone marrow mast cells
(BMMCs) of wild-type and Lyn/ mice (14) were provided by
Dr. M. Hibbs (Ludwig Institute for Cancer Research, Melbourne, Australia). B cell line Ramos, myeloid cell line THP-1,
T cell line Jurkat, and 293T cells were from the cell line collection of the Institute of Molecular Genetics. Jurkat T cell line mutant J.CaM2.5 deficient in the transmembrane adaptor protein
LAT (15) was donated by Dr. A. Weiss (University of California
at San Francisco, San Francisco, CA).
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Antiserum to non–T cell activation linker (NTAL) was produced in the Prague laboratory by immunization of rabbits with
bacterially expressed cytoplasmic fragment of human NTAL
(amino acids 89–243), mouse mAbs to NTAL were prepared using standard techniques from splenocytes of mice immunized
with the same bacterially produced NTAL fragment; some of
them cross-reacted also with mouse homologue (unpublished
data). In addition, anti-peptide mAbs were prepared directed to
the peptide comprising residues 196 to 210 of the human molecule (purchased from Genemed Synthesis Inc.) conjugated to
keyhole limpet hemocyanin using a commercial kit (Pierce
Chemical Co.). Rabbit polyclonal antibodies to Erk1/2 and
phospho-Erk1/2 were from Promega and New England Biolabs,
Inc., respectively.
The sources of the other antibodies used were as follows: Jurkat TCR (IgM mAb C305, provided by Dr. A. Weiss), CD28
(IgM mAb 248.23.2; reference 16), Grb2 (mouse mAb; Transduction Laboratories), Sos1 (rabbit polyclonal; Santa Cruz Biotechnology, Inc.), c-Cbl (rabbit polyclonal; Santa Cruz Biotechnology, Inc.), Gab1 (Upstate Biotechnology), ubiquitin (rabbit
polyclonal; Sigma-Aldrich), phosphotyrosine (mAbs P-Tyr-01
and P-Tyr-02 prepared in the Prague laboratory; PY-20-horseradish peroxidase conjugate; Transduction Laboratories), CD59
(mAb MEM-43), and CD3 (mAb MEM-92), both prepared in
Prague laboratory.
In Vitro Kinase Assay, Immunoprecipitation, and Other Biochemical
Methods. Detergent-resistant microdomains (GEMs) were immunoisolated and in vitro kinase assays performed as described
previously (17). Briefly, cells were solubilized with ice-cold isotonic lysis solution containing 1% detergent Nonidet P-40 (NP40), the postnuclear supernatants containing GEMs were incubated in plastic wells coated with mAb MEM-43 (antibody to a
major protein component of GEMs, GPI-anchored protein CD59). After washing, the kinase solution containing
[32P]ATP (ICN Biomedicals) was added and the proteins
phosphorylated by GEMs-associated kinases were resolved by
SDS-PAGE and autoradiography. Sucrose density gradient-based
GEMs isolation was performed as described previously (18).
NTAL and NTAL-containing complexes were immunoprecipitated using postnuclear supernatants of cells solubilized by a
detergent effectively disrupting GEMs (laurylmaltoside [n-dodecyl -D-maltoside]; Calbiochem; lysis buffer: 1% laurylmaltoside
in 20 mM Tris [pH 7.5], containing 100 mM NaCl, 10% glycerol, 1 mM 4-(2-aminoethyl)-benzenesulfonyl fluoride, 10 mM
EDTA, 50 mM NaF, 1 mM Na3VO4) and CNBr-Sepharose
beads (Amersham Biosciences) coupled with mAbs purified by
Protein A-Sepharose affinity chromatography. These lysates were
passed through minicolumns (30–50 l packed volume) of such
immunosorbents; after washing with 10 column volumes of lysis
buffer, bound proteins were eluted with 2 column volumes of
2 concentrated SDS-sample buffer and the flow-through and
eluted fractions were analyzed by SDS-PAGE followed by Western blotting. In some experiments the cells were solubilized in
2 concentrated SDS-sample buffer, ultracentrifuged (250,000 g,
30 min) and the supernatant analyzed by SDS-PAGE and Western blotting. Biosynthetic labeling with [3H]palmitate, SDSPAGE, and Western blotting were performed as described (18).
For enhanced detection of polyubiquitinated NTAL, the blot was
autoclaved before immunostaining (19).
Isolation of NTAL and Its Identification by Peptide Mass Mapping. Large-scale isolation of GEMs from NP-40-solubilized
THP-1 cells by sucrose density gradient ultracentrifugation and
separation of their protein components by two-dimensional
Novel Transmembrane Adaptor Protein
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ing molecules such as phospholipase C (PLC), phosphatidylinositol 3-kinase (PI3-K), and other proteins
regulating activities of small G-proteins of Ras and Rho
families. A key component of the TCR signaling pathway
is a transmembrane adaptor protein linker for activation of
T cells (LAT) which becomes tyrosine phosphorylated by
activated ZAP-70 (4) and then binds several other molecules including PLC, Grb2, SLP-76, PI3-K, and Gads (5).
LAT is expressed in T cells as a palmitoylated protein and is
a characteristic component of detergent-resistant membrane microdomains (6, 7), also called membrane rafts or
glycosphingolipid-enriched membrane domains (GEMs)
enriched in glycosylphosphatidylinositol (GPI)-anchored
proteins, glycosphingolipids, cholesterol, and several species of signaling molecules including Src-family kinases and
heterotrimeric G-proteins (8). Association of the ligated
TCR (as well as other immunoreceptors) with GEMs
seems to be indispensable for the initiation of cellular activation as it facilitates the phosphorylation of immunoreceptor-associated tyrosine-based activation motifs by GEMassociated Src-kinases and several other early signaling steps
(9, 10). T cells devoid of LAT are defective in TCR signaling and LAT/ mice lack mature T cells as their development in thymus is blocked at an early stage (11).
In marked contrast to T cells, B cells of LAT/ mice
are functionally normal because LAT is not expressed in
these cells. Furthermore, myeloid and NK cells develop
apparently normally in the LAT-deficient animals and at
least some aspects of signaling through their Fc-receptors
remain functional (11–13). The latter data indicate that
another LAT-like molecule may be expressed in non–T
cells. Therefore, we were looking for a molecule which
might possibly play a LAT-like role in BCR and Fc-receptor signaling.
Published December 16, 2002
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Brdička et al.
expression of CD3 as the parental J.CaM2.5 cells or wild-type
Jurkat cells (unpublished data). In relevant figures a representative
of the NTAL high-expressing clones (which all gave similar results) is compared with wild-type Jurkat cells and to a parental
J.CaM2.5 cell line (which behaves essentially identically as the
NTAL-low expressing clone).
Mouse Genomic Clones. LAT genomic clones were isolated
from a 129/Ola phage library and sequenced. The GenBank/
EMBL/DDBJ accession no. corresponding to mouse LAT is
AJ438435. Part of the sequence of the mouse LAT gene can be
found on a contig present in the Ensembl Gene report (Ensembl
gene ID: ENSMUSG00000030742; http://www.ensembl.org/
Mus_musculus/), allowing the location of the LAT gene to chromosome 7.
Confocal Microscopy. THP-1 cells and J.CaM2.5-NTAL
transfectants were spun on coverslips coated with poly-L-lysine
(Sigma-Aldrich), fixed, and permeabilized 3 min in 20 C
methanol and then 5 s in cold acetone. After washing in PBS
the slides were blocked with PBS containing 1% bovine serum
albumin and 20% human AB serum and incubated for 45 min
with mouse mAb to NTAL (NAP-7, 50 g/ml), followed by
45 min incubation with Alexa 488 goat anti–mouse IgG (Molecular Probes, 500 diluted). Nuclei were stained with propidium iodide (10 min, 0.5 g/ml). The samples were mounted
in PBS and viewed with a Laserscan microscope (Leica TCS
SP). Incubation with irrelevant primary antibody served as a
negative control.
Tissue Section Immunostaining. Sample of intestinal tissue biopsy from a colorectal carcinoma patient (including a normal tissue with local lymph nodes) was fixed with 10% neutral buffered
formalin, embedded into parafin, and 4-m thick tissue sections
were cut. The preparation was dipped into citrate buffer pH 6.0
and treated in a microwave oven (2 5 min; 750 W). After
blocking endogenous peroxidase activity by 1.5% H2O2 in methanol for 20 min, tissue sections were incubated sequentially with
hybridoma supernatant containing anti-NTAL mouse monoclonal antibody, biotinylated anti–mouse antibody (Jackson ImmunoResearch Laboratories), streptavidin-conjugated horseradish peroxidase (Biogenex), and 3,3-diaminobenzidine.
Cell Activation. THP-1 cells were incubated 30 min on ice
with an irrelevant mouse IgG2a monoclonal antibody (50 g/ml
in HBSS) which binds in the monomeric form selectively to the
human high affinity IgG receptor (FcRI; CD64; reference 22)
and then 20 min at 37 C in culture medium. Ligated Fc-receptors were then cross-linked with polyclonal goat anti–mouse antibody (Sigma-Aldrich; 20 g/ml; 2 min at 37 C), cooled down
in ice-water bath for 1 min, spun down 1 min at 2 C, and immediately detergent solubilized. In some experiments THP-1 cells
were stimulated in the presence of kinase inhibitors. Purified
monocytes were stimulated using a similar protocol, except that
they were first incubated with human AB serum and the ligated
Fc-receptors were then cross-linked with polyclonal rabbit anti–
human Ig antibody (Jackson ImmunoResearch Laboratories).
BMMCs from wild-type mice or from mice with a genetically
disrupted Lyn gene (BMMC-Lyn/) were sensitized with monoclonal IgE (IGEL b4 1; 1 g/ml) and the ligated FcRIs were aggregated with 2,4,6-trinitrophenyl (TNP)-BSA conjugate (1 g/
ml; 5 min at 37 C) as described elsewhere (23). Ramos B cells
were activated by incubation for 2 min at 37 C with F(ab)2 fragments of goat anti–human IgM (Jackson ImmunoResearch Laboratories). Mouse B cells present in the unseparated splenocyte suspension (108 cells/ml) were stimulated 30 s with F(ab)2 fragments
of goat anti–mouse IgM (20 g/ml; Jackson ImmunoResearch
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PAGE was performed essentially as described earlier for analogous
isolation of the transmembrane adaptor protein PAG (18). Two
silver stained spots comigrating with the radioactively labeled in
vitro phosphorylated spots were digested directly in the gel by
trypsin (Promega) and the resulting peptide mixtures were analyzed on a Bruker BIFLEX II (Bruker-Franzen) MALDI-TOF
mass spectrometer equipped with a nitrogen laser (337 nm) and a
delayed extraction ion source. A saturated solution of -cyano4-hydroxycinnamic acid in aqueous 50% acetonitrile and 1%
acetic acid was used as a MALDI matrix. 1 l of the sample and 1
l of the matrix solution were mixed on the target and allowed
to dry at the ambient temperature. Positive-ion mass spectra of
peptide maps were measured in the reflectron mode. Spectra
were externally calibrated by using the monoisotopic [M H]
ion of peptide standard (somatostatin; Sigma-Aldrich).
DNA Constructs, Transfections. The coding region of human
NTAL was amplified from human leukocyte cDNA library
(CLONTECH Laboratories, Inc.; primers: 5 CAGTTCTTGGAAACCCACTCGAG 3 and 5 GATGTCGACTAGGCTTCTGTGGCTGCCAC 3 ). The PCR product was blunted and
cloned into EcoRV site of pBluescript SK vector (Stratagene), the
coding sequence was then cut out with HindIII and SmaI, gelpurified, and cloned into the HindIII/EcoRV digested eukaryotic
expression vector pFLAG-CMV 5a (Sigma-Aldrich), and sequenced. For stable transfections NTAL coding sequence was
subcloned into EcoRI site of pEFIRES-N vector (provided by
Dr. S. Hobbs, Institute of Cancer Research, London, UK; reference 20). The FLAG-NTAL construct encoding the full length
NTAL containing the COOH-terminal FLAG tag was produced
from the pFLAG-CMV 5a construct by site-directed mutagenesis
using the QuikChange™ site-directed mutagenesis kit (Stratagene) according to the manufacturer’s instructions and the
FLAG-NTAL insert was eventually subcloned into the pEFIRES-N vector. The analogous FLAG-LAT construct was obtained from a previously described construct (21) by subcloning
the FLAG-LAT insert into the pEFIRES-N vector. The FLAGTRIM construct in pEF-BOS vector was described earlier (21).
For transient transfection of 293T cells, Lipofectamine 2000™
reagent (Invitrogen) was used according to manufacturers instructions. For transfection experiments in 293T cells the following
cDNA constructs were used: Myc-tagged Lck or ZAP-70 inserted into pcDNA3 vector (donated by Dr. R. Abraham, Mayo
Clinic, Rochester, MN), Syk cloned into the pRK5 vector
(provided by Dr. W. Kolanus, Gene Center, Munich, Germany),
Myc-tagged Lyn in pcDNA3.1 vector (provided by Dr. S. Watson, University of Oxford, UK), and FLAG-tagged Hck in
pcDNA1 vector (provided by Dr. G. Langsley, Institut Pasteur,
Paris, France). For bacterial expression, the NTAL intracellular
fragment corresponding to amino acids 89–243 was cloned to
BamHI site of pET-15b expression vector (Novagen), generating
a construct with NH2-terminal histidine tag.
J.CaM2.5 cells (Jurkat T cell line mutant deficient in the transmembrane adaptor protein LAT) were transfected with nontagged or FLAG-tagged NTAL, LAT, and TRIM constructs in
the above described expression vectors by electroporation; stable
transfectants expressing NTAL were selected by growing in 96well plate in selective medium containing 1 mg/ml G418 (Calbiochem). After 3 wk oligoclonal G418-resistant populations were
expanded and checked for NTAL expression by Western blotting. Four independent clones (three expressing high amount of
NTAL and one of very low expression) were analyzed in detail.
All these clones expressed only trace amounts of LAT (just like
the parental J.CaM2.5 cells) and all of them had the same high
Published December 16, 2002
Results
NTAL Is a Transmembrane Adaptor Protein Similar to
LAT. In vitro kinase assays performed on GEMs immunoprecipitated from myeloid cell lines HL-60 and THP-1
revealed the presence of an unidentified 30 kD phosphoprotein (pp30) which was not detectable under similar
conditions in T cells (Fig. 1 A). To further characterize this
protein, the in vitro labeled proteins were mixed with
GEMs prepared from 5 108 THP-1 cells as described
previously (18). This mixture was then subjected to twodimensional gel electrophoresis and a doublet of acidic
protein spots of 29–30 kD visualized by silver staining was
found to colocalize with radiolabeled pp30 (Fig. 1 B). The
spots were excised, digested in-gel with trypsin, and resulting peptides were analyzed by mass spectrometry
(MALDI-TOF). Database searching revealed that six of the
peptides (Table I) fit precisely to those predicted for a so far
uncharacterized protein encoded by a previously cloned
full-length cDNA corresponding to a broadly expressed
human gene termed WBSCR5 which is located on human
chromosome 7 (7q11.23; references 24 and 25). The
WBSCR5 cDNA codes for a polypeptide of 243 amino
acid residues (Fig. 2) and a predicted molecular weight of
26.550 daltons while the predicted mouse homologue is
shorter by 40 amino acid residues and is encoded by a gene
residing on chromosome 5 (25). The protein resembles in
its general organization the GEM-associated transmembrane adaptor proteins PAG/Cbp (18, 26) and LAT (4).
Thus, it consists of a very short NH2-terminal extracellular
peptide (6aa), a single putative hydrophobic transmembrane domain which is followed by a potential palmitoylation site (a CxxC motif). The predicted cytoplasmic domain contains a total of 10 tyrosines but no other clearly
recognizable motifs. Importantly, the mouse LAT and
WBSCR5 genes show a strikingly similar organization.
Both of them are composed of 11 exons that split the respective coding sequence in a very similar manner (Fig. 3).
The two genes (residing on different chromosomes) also
display the same splice frame diagrams (Fig. 3) further suggesting that they are closely related and likely are derived
from a common ancestor (see Discussion). Because of the
structural similarity to LAT and its broad expression we
termed to the novel protein NTAL.
Based on the published cDNA sequence of human
WBSCR5, a full length NTAL cDNA was obtained by
PCR from human leukocyte cDNA library and subcloned
into bacterial (pET-15b) or eukaryotic (pEFIRES-N) expression vectors. Rabbit polyclonal and mouse monoclonal
antibodies raised to the bacterially produced major part of
the cytoplasmic domain of NTAL detected a band of the
appropriate size in the LAT-deficient Jurkat variant
JCaM2.5 following expression of NTAL (Fig. 4 A).
Western blotting further demonstrated absence of
NTAL in peripheral blood T cells, moderate expression in
monocytes, and strong expression in peripheral blood B
lymphocytes and NK cells (Fig. 4 B); NTAL is also strongly
expressed in B cell lines Raji and Ramos and myeloid lines
Figure 1. NTAL isolation. (A) Proteins
of myeloid cell (HL-60) vs. T cell (Jurkat)
membrane microdomains (GEMs) phosphorylated under the conditions of the in
vitro kinase assay; a pattern very similar to
that of HL-60 was observed also in the case
of the THP-1 cell preparation (unpublished
data). (B) Proteins of THP-1 GEMs were
mixed with the preparation obtained by in
vitro kinase assay, separated by 2-dimensional gel electrophoresis and detected by
silver staining or autoradiography. Arrows
indicate the position of pp30.
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Novel Transmembrane Adaptor Protein
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Laboratories). In vitro activation of Jurkat T cells, J.CaM2.5 mutants, and J.CaM2.5-NTAL stable transfectants was performed using soluble IgM anti-CD3 mAb MEM-92 (100–250 diluted ascitic fluid containing approximately 8 mg/ml mAb, at 37 C for 5
min). Activated (phosphorylated) Erk1/2 was determined by immunoblotting of the total cell lysates using phospho-Erk specific
antibody (New England Biolabs, Inc.). J.CaM2.5 cells transiently
transfected with the FLAG-tagged LAT, NTAL, or TRIM constructs were 18 h after the transfection stimulated for 2 min with a
combination of anti-TCR (C305) and anti-CD28 IgM mAbs
(hybridoma supernatants) and phospho-Erk1/2 and FLAG
epitope were determined in their detergent lysates by Western
blotting.
Flow Cytometry Analysis of Calcium Mobilization. Jurkat,
J.CaM2.5, and J.CaM2.5-NTAL cells were loaded with fluorescent Ca2 indicators Fura Red and Fluo-4 (9.2 M and 3.6 M,
respectively; Molecular Probes) in HBSS containing 10 mM
HEPES (Sigma-Aldrich) and 4 mM Probenecid (Sigma-Aldrich),
for 20 min in dark and at room temperature. The cells were
washed twice in HBSS containing 10 mM HEPES and 1% fetal
calf serum (HBSS/FCS), resuspended to final concentration of
106 per ml, rested for 15 min in dark, and preheated for 15 min at
37 C before the measurement performed at 37 C. After 1 min,
anti-CD3 (MEM-92) mAb (100 diluted ascitic fluid containing
approximately 8 mg/ml mAb) at 10 g/ml final concentration
was added and the measurement was continued for additional 5
min. Finally, adequacy of cellular loading was verified by treating
the cells with ionomycin (Sigma-Aldrich; 2 g/ml final concentration). Data were acquired on FACSort™ flow cytometer (Becton Dickinson) at 500 ms time points and ratiometric analysis was
performed with Flow Jo software (Tree Star).
Published December 16, 2002
Table I. Tryptic Peptides Identified by Mass Spectrometry
Position of peptide
in the polypeptide
chain
Calculated masses
[M H]
1,677.7
1,722.8
1,965.9
2,131.0
1,677.8
1,722.9
1,966.0
2,131.1
2,147.0
2,147.1
2,163.0
2,259.0
2,163.1
2,259.1
2,275.0
2,275.1
2,291.0
2,400.9
2,291.1
2,401.0
2,416.9
2,417.0
2,432.9
2,433.0
The six peptides and their derivatives shown in the table fit
unequivocally to the WBSCR5 gene product (AAF74978) and covered
30% of its predicted sequence.
aCarbamidomethylation of cysteine (due to iodoacetamide present in
the lysis buffer).
bMono-oxidation of methionine.
cDi-oxidation of methione.
THP-1 and HL-60 but not in T cell lines HPB-ALL and
Jurkat (unpublished data). Immunohistochemical staining
of paraffin tissue sections revealed a particularly strong expression in germinal centers of human lymph nodes (Fig. 5
A). As expected, NTAL is mostly present in buoyant
GEMs (Fig. 5 B), it can be biosynthetically labeled by 3H-
Figure 2. Predicted amino acid sequence of human NTAL (product of
the WBSCR5 gene). The putative transmembrane region is boxed, the
potential palmitoylation sequence, the tyrosine-x-asparagine motifs, and
all other tyrosines are in bold and underlined. These sequence data are
available from GenBank/EMBL/DDBJ under accession no. AAF74978.
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Brdička et al.
Figure 3. Comparison of the exon-intron organization and of the
splice frame diagrams of the mouse genes encoding LAT and NTAL, respectively. Exons are shown by boxes; the positions of the initiation
(Start) and termination (Stop) codons are indicated by vertical arrows.
Based on splice frame junctions, three types of introns can be distinguished in a given gene: phase 0 intron interrupts the reading frame between two consecutive codons, whereas phase 1 and phase 2 introns interrupt the reading frame between the first and the second nucleotide of a
codon or between the second and the third nucleotide of a codon, respectively (reference 40). According to that classification, the phase class
of each intron is indicated by a solid circle on the diagram shown below
each gene. For the sake of clarity, the length of introns is not drawn to
scale. The structure of the mouse NTAL (WBSCR5) gene is reported in
(reference 25) and that of LAT in this paper.
palmitate (Fig. 5 C), and clearly localizes to the plasma
membrane (Fig. 5 D).
NTAL Is Tyrosine-phosphorylated after FcRI, FcRI, or
BCR Cross-linking and Becomes Associated with Other Signaling Proteins. The overall similarity of NTAL to LAT and
the results of in vitro kinase assay indicated that NTAL
might be inducibly tyrosine-phosphorylated after triggering
of immunoreceptors. Indeed, NTAL became tyrosinephosphorylated and associated with additional phosphoproteins following cross-linking of the high-affinity IgGreceptor (FcRI/CD64) on human THP-1 myeloid cells and
blood monocytes, the high-affinity IgE-receptor (FcRI)
on mouse BMMCs, and the BCR on human Ramos and
mouse splenic B cells (Fig. 6 A).
The major proteins inducibly associating with NTAL in
cells activated via the BCR or the FcRI were identified as
Figure 4. Expression of NTAL. (A) cDNA encoding human NTAL
was expressed in J.CaM2.5 cells and the protein product was visualized by
Western blotting of the transfectants detergent lysate as compared with
Ramos cells (expressing endogenous NTAL). (B) Western blotting of the
indicated subpopulations of human peripheral blood cells (immunostaining for NTAL or Erk; the latter was used as a loading control).
Downloaded from jem.rupress.org on January 5, 2016
146-161
C2H3ON@Cysa
80–94
78–94
57–76
57-76
O@Metb
57-76
20@Metc
57–77
57-77
O@Metb
57-77
2O@Metc
104–123
104-123
O@Metb
104-123
2O@Metc
Measured masses
[M H]
Published December 16, 2002
Grb2, Sos1, and Gab1 (in both Ramos and THP-1 cells)
and c-Cbl (only in THP-1; Fig. 6 B). A fraction of the
NTAL protein immunoprecipitated from detergent lysate
of anti-BCR–stimulated Ramos cells exhibited strongly
decreased electrophoretic mobility indicative of possible attachment of multiple ubiquitin residues. This was confirmed directly by Western blotting of NTAL immunoprecipitates prepared from BCR-triggered Ramos cells (Fig. 6
C). Maximum level of NTAL tyrosine phosphorylation in
Ramos cells was observed already after 15 s of stimulation,
whereas the maximal ubiquitinylation was seen after 3
min (unpublished data).
To determine which protein tyrosine kinases (PTKs) are
able to phosphorylate NTAL, we treated THP-1 cells with
the Src-family PTK inhibitor PP2 or the Syk-family PTK
inhibitor piceatannol and then stimulated them via FcRI
cross-linking. As shown in Fig. 7 A, both inhibitors suppressed tyrosine phosphorylation of NTAL. Coexpression
of NTAL with Src-family kinases Lck, Lyn, Hck, or Yes
and/or Syk or ZAP-70 in 293T-cells indicated that NTAL
was, similarly as LAT, most strongly phosphorylated in the
presence of simultaneously expressed Lck and ZAP-70 or
Lck and Syk (Fig. 7 B). Furthermore, no tyrosine phosphorylation of NTAL was observed in Lyn/ mouse BMMCs
1622
Figure 6. Induction of NTAL tyrosine phosphorylation and association
with cytoplasmic signaling proteins. (A) THP-1 cells or purified human
monocytes were stimulated via their FcRI receptors, Ramos cells or
murine B lymphocytes via BCR, and mouse BMMC via FcRI receptors. NTAL was immunoprecipitated from unstimulated () or stimulated ( ) cells and analyzed by SDS-PAGE and Western blotting using
anti-phosphotyrosine antibody to visualize tyrosine-phosphorylated
NTAL (top panel). The bottom panel represents immunostaining of
NTAL in the same samples and in the same position of the blot (around
30 kD). (B) The same NTAL immunoprecipitates as shown in part A
were analyzed by Western blotting using antibodies to the indicated associated molecules. (C) Blots of NTAL immunoprecipitates from unstimulated () or anti-BCR-stimulated ( ) Ramos cells were immunostained
by antibodies to NTAL or ubiquitin (Ubq.). Only the relevant parts of
the blots are shown in parts A and B, corresponding to the size of the
relevant proteins.
stimulated via FcRI (compare Fig. 6 A for the wild-type
BMMCs) indicating that Lyn is directly or indirectly responsible for NTAL inducible phosphorylation in these
cells (unpublished data).
Expression of NTAL Can Partially Compensate for LAT
Deficiency in T Cells. To assess whether NTAL could exert a LAT-like function, NTAL was stably expressed in the
LAT-deficient Jurkat variant J.CaM2.5 (which also does
not express NTAL). After stimulation of the transfectants
with an agonistic CD3 mAb, NTAL became rapidly tyrosine-phosphorylated and associated with Grb2, Sos1, and
c-Cbl (Fig. 8 A) but not with SLP-76 or PLC1 (unpublished data). CD3 stimulation was accompanied by a minimal (but reproducible) and transient increase in cytoplasmic
calcium level (Fig. 8 B) and a partial rescue of Erk1/2
phosphorylation (Fig. 8 C). To compare semiquantitatively
the effects of NTAL expression with that of LAT,
J.CaM2.5 cells were transiently transfected with expression
constructs encoding FLAG-tagged LAT, NTAL, TRIM,
or with the vector only and activation of Erk1/2 was followed after anti-CD3 plus anti-CD28 stimulation. As
shown in Fig. 8 D, NTAL partially rescued activation of
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Figure 5. Tissue and subcellular localization of NTAL. (A) Paraffin
section of lymphoid tissue immunoperoxidase stained for NTAL; the major positive structures are germinal centers. (B) Localization of NTAL in
buoyant detergent-resistant microdomains (GEMs). THP-1 cells were
solubilized in the presence of 3% nonionic detergent Brij-58 or 1% laurylmaltoside (LM; a detergent known to disrupt GEMs) and subjected to
sucrose density gradient ultracentrifugation; the fractions (numbered from
top to bottom) were analyzed by Western blotting. (C) Biosynthetic labeling of NTAL with [3H]palmitate; NTAL immunoprecipitate was analyzed by SDS-PAGE followed by fluorography of the gel. (D) Plasma
membrane localization of NTAL (green) as determined by confocal microscopy in THP-1 cells and J.CaM2.5-NTAL transfectants; nuclei are
shown in red.
Published December 16, 2002
Erk1/2 when expressed at a level comparable to that of
LAT under the same conditions while expression of TRIM
had no effects even at much higher expression level. Thus,
the ectopically expressed NTAL can partially restore at least
some aspects of TCR signaling in LAT-deficient mutants.
Discussion
The new transmembrane adaptor protein NTAL described in this paper (a product of the previously described
gene WBSCR5; references 24 and 25) appears to be structurally closely related to the critical component of the
TCR signaling pathway, LAT (4). Moreover, the organization of the genes encoding LAT and NTAL, respectively, is
also similar, indicating they probably have a common evolutionary origin. Interestingly, the expression pattern of
NTAL in lymphocytes is largely complementary to that of
LAT: while LAT is predominantly found in T but not B
lymphocytes, the reverse is true for NTAL. Among the important features of the structure of NTAL is a potential
palmitoylation site (CxxC) adjacent to the transmembrane
domain that is presumably responsible for targeting the
protein to membrane microdomains (rafts, GEMs). Furthermore, there are five potential Grb2 binding motifs
(YxN). Our data using PTK inhibitors (Fig. 7) and coexpression of NTAL and PTKs in 293T cells indicate that
NTAL, again similarly to LAT, is phosphorylated by con1623
Brdička et al.
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Figure 7. Kinases phosphorylating NTAL. (A) Inhibition of NTAL tyrosine phosphorylation in THP-1 cells stimulated via FcRI by the indicated PTK inhibitors. NTAL immunoprecipitates prepared from the
treated and control cells were analyzed by Western blotting to detect P-Tyr
or NTAL, respectively. (B) Phosphorylation of NTAL coexpressed in
293T cells with various Src- and Syk-family kinases. Total cell lysates
were analyzed by SDS-PAGE and Western blotting to detect the indicated molecules.
certed action of Src- and Syk-family kinases (presumably
the Src-family kinases are needed for activation of the Syk
family kinases).
Induction of NTAL tyrosine phosphorylation after BCR
cross-linking is reminiscent of the phosphorylation of LAT
that is induced by TCR ligation (4). Similar to LAT in
activated T cells, NTAL immunoprecipitated from BCRstimulated B lymphocytes is associated with the cytoplasmic linker protein Grb2 and the nucleotide exchange factor of the small G-protein Ras, Sos1 (27). Surprisingly, and
in clear contrast to LAT, we never observed an inducible
association of NTAL with PLC or another cytoplasmic
adaptor protein, SLP-76 (or its B cell analogue SLP-65/
BLNK; references 28 and 29), both of which are key components of the multicomponent complex that is organized
by activated (tyrosine-phosphorylated) LAT (5). This could
suggest that the role of NTAL in BCR signaling differs
from that of LAT in the TCR signaling; namely, that
NTAL may be involved only in activation of the Grb2/
Sos1-initiated pathway(s) but not in activation of the
PLC-Ca2 pathway. The finding of the adaptor protein
Gab1 in the NTAL immunoprecipitates may further suggest that the NTAL-Grb2-Gab1 complex possibly regulates
the activity of PI3-K in stimulated cells (30–32), but this
remains a speculation at this moment.
The lack of association of SLP-65 and PLC with phosphorylated NTAL may further indicate that SLP-65 does
not require a LAT-like molecule in B cells for being targeted to the plasma membrane. Indeed, recent data suggested that SLP-65 binds directly with its Src-homology 2
(SH2)-domain to a highly conserved non-ITAM tyrosine
motif within the cytoplasmic domain of CD79a (Ig; references 33 and 34). Moreover in contrast to SLP-76, phosphorylated SLP-65 binds SH2 domains of PLC2 (35).
Thus, it is tempting to speculate that the multiple functions
that LAT exerts in T cells are shared in B lymphocytes between NTAL and other molecules, for example, CD79a.
Similarly to the situation in B cells, NTAL becomes
strongly tyrosine phosphorylated after cross-linking of FcRI and FcRI and then associates with Grb2 and Sos1
(but again not with PLC and also not with SLP-76).
Thus, also in these cells NTAL seems to be involved in
linking the activated immunoreceptors to the Grb2/Sos
pathway. In contrast to activated B cells, NTAL also interacts with c-Cbl in activated THP-1 cells (Fig. 6). LAT has
been shown to be important for processing of the FcRI
and FcRI mediated signals; however, FcRI and FcRI
signaling in LAT/ cells is still partially functional (12,
13). It is therefore tempting to speculate that this residual
Fc-receptor signaling capacity in myeloid cells is due to the
presence of NTAL. It is important to note that FcRI
(CD64) is the only human Fc-receptor that binds soluble
monomeric murine antibodies of the IgG2a isotype with
sufficient affinity (22). This indicates that under the used
experimental conditions only this Fc-receptor became activated in our experiments with the THP-1 cells. Furthermore, an identical pattern of NTAL phosphorylation was
observed in the THP-1 cells when FcRI (CD64) was di-
Published December 16, 2002
rectly cross-linked by a CD64-specific monoclonal antibody; in contrast, no phosphorylation was observed after
direct mAb-mediated cross-linking of FcRII (CD32) or
FcRIII (CD16) in these cells (unpublished data).
Our experiments provide preliminary evidence for a
functional LAT-like role of NTAL in immunoreceptor signaling: ectopic expression of this protein in LAT-deficient
J.CaM2.5 Jurkat T cells partially rescues TCR/CD3-mediated signaling, namely activation of Erk1/2 (Fig. 8). The
minimal calcium response accompanying the CD3-mediated stimulation may be in agreement with the observed
lack of coprecipitation of PLC and SLP-76 with activated
NTAL in these transfectants, as discussed above. The striking conservation of the exon-intron organization of the
genes encoding LAT and NTAL, respectively (Fig. 3), suggests that they probably derive from a duplication of an ancestral gene. As reported for other gene families, in the
course of evolution, the original position of the exon borders has been blurred by splice junction sliding (36). It is
1624
important to note that the exon-intron organization and
splice frame diagram of genes encoding other transmembrane adaptor proteins involved in immunoreceptor signaling, e.g., SIT (37) or TRIM (38) differ totally from the distinctive organization found in the genes enconing LAT and
NTAL. A similar relationship of gene organization was
previously noted for the functionally closely related signal
transducing subunits of several immunoreceptors (39).
Thus, NTAL appears to be structurally, evolutionarily and
probably also functionally related to the transmembrane
adaptor protein LAT.
We would like to thank Drs. R. Abraham, M. Hibbs, S. Hobbs,
W. Kolanus, G. Langsley, S. Watson, and A. Weiss for kindly providing us with cells and constructs as specified in Materials and
Methods.
This work was supported from the projects Center of Molecular
and Cellular Immunology (LN00A026) and grant no. 1131000001
from Ministry of Education, Youth and Sports of the Czech Re-
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Figure 8. Functional analysis of NTAL in LAT-defective J.CaM2.5 transfectants. (A) NTAL immunoprecipitates obtained from unstimulated () or
anti-CD3 stimulated ( ) J.CaM2.5 mutants and J.CaM2.5-NTAL transfectants were analyzed by Western blotting for the presence of the indicated molecules. The top panel corresponds to tyrosine-phosphorylated NTAL (30 kD). (B) Wild-type Jurkat, J.CaM2.5, and J.CaM2.5-NTAL transfectants were
stimulated by anti-CD3 IgM mAb (added at time points indicated by arrows) and increase of cytoplasmic Ca2 was measured. (C) Wild-type Jurkat,
J.CaM2.5, and J.CaM2.5-NTAL transfectants were stimulated by optimally diluted anti-CD3 IgM mAb and after 5 min of activation Erk1/2 was detected in the cell lysates by Western blotting using anti-phospho Erk antibody; bottom panel represents control staining by anti-Erk. (D) J.CaM2.5 cells
transiently transfected with the indicated FLAG-tagged constructs were stimulated for 2 min by anti-CD3 and anti-CD28 mAbs and activation of Erk1/2
was detected as in C (top panel); presence of equal amounts of Erk1/2 in all samples was ascertained (middle panel) and the level of expression of individual FLAG-tagged proteins was determined (bottom panel).
Published December 16, 2002
public, grant J1116W24Z from the Wellcome Trust (to V.
Hoř ej š í), Research Center Immunology Magdeburg/Halle, Sachsen-Anhalt (supported by grant 01 ZZ 0110 of the Bundesministerium für Bildung und Forschung to B. Schraven); P. Dráber is supported by an International Research Scholar’s Award from Howard
Hughes Medical Institute, M. Malissen is supported by Centre National de la Recherche Scientifique and Institut National de la Sante
et de la Recherche Medicale, and E. Aguado was supported by a
fellowship from the European Communities.
15.
16.
17.
Submitted: 13 August 2002
Revised: 30 October 2002
Accepted: 6 November 2002
18.
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