THE JOURNAL OF BIOLOGICAL CHEMISTRY
© 2002 by The American Society for Biochemistry and Molecular Biology, Inc.
Vol. 277, No. 25, Issue of June 21, pp. 22520 –22527, 2002
Printed in U.S.A.
Interchangeable Enzyme Modules
FUNCTIONAL REPLACEMENT OF THE ESSENTIAL LINKER OF THE BIOTINYLATED SUBUNIT OF ACETYLCoA CARBOXYLASE WITH A LINKER FROM THE LIPOYLATED SUBUNIT OF PYRUVATE DEHYDROGENASE*
Received for publication, February 6, 2002, and in revised form, March 29, 2002
Published, JBC Papers in Press, April 15, 2002, DOI 10.1074/jbc.M201249200
John E. Cronan, Jr.‡
From the Departments of Microbiology and Biochemistry, University of Illinois, Urbana, Illinois 61801
* This work was supported in part by National Institutes of Health
Grant AI15650. The costs of publication of this article were defrayed in
part by the payment of page charges. This article must therefore be
hereby marked “advertisement” in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
‡ To whom correspondence may be addressed: Dept. of Microbiology,
University of Illinois, B103 Chemical and Life Sciences Laboratory, 601
S. Goodwin Ave., Urbana, IL 61801. Tel.: 217-333-7919; Fax: 217-2446697; E-mail: j-cronan@life.uiuc.edu.
Biotin and lipoic acid are vitamins that play essential roles
in central metabolism. The biological activities of both coenzymes are dependent upon their covalent attachment to their
cognate proteins (1). In both cases, the site of coenzyme attachment is the ⑀-amino group of a lysine residue centrally located
within conserved sequences of ⬃70 amino acid residues that
fold to form discrete protein domains. The three-dimensional
structures of both biotinoyl (2– 6) and lipoyl domains (7–9) from
several biological sources have been determined, and the structures are largely superimposable. Indeed, Reche and Perham
(10) have succeeded in altering the Escherichia coli acetyl-CoA
carboxylase (ACC)1 biotinoyl domain such that the normally
absolutely specific lipoate protein ligase will attach lipoic acid
to various mutant biotinoyl domains. Therefore, it seems clear
that two enzyme families that catalyze very different reactions
use a common structural domain to carry the essential coenzyme. Biotinylated enzymes catalyze carboxylation and decarboxylation reactions and play essential roles in fatty acid synthesis and amino acid degradation, whereas lipoylated
enzymes catalyze acyl transfer and single carbon transfer reactions and are required for function of the citric acid and
glycine cleavage cycles (1). Both the biotinoyl and lipoyl domains are thought to act as swinging arms that convey covalently bound intermediates between different active sites of a
multienzyme complex (1). Swinging arm mobility has two different aspects. Mobility on a small scale is imparted by attachment of the carboxyl of the coenzyme to the ⑀-amino group of a
lysine residue located at the tip of a protruding -turn. As first
proposed (see Ref. 11) this arrangement gives the “business
ends” of the coenzymes a significant reach. Mobility on a much
larger scale is imparted by the proline plus alanine-rich sequences adjacent to the lipoyl domains that act as flexible
linkers (Fig. 1) (1). In lipoyl enzymes having only a single
domain, the domain forms the amino terminus of the protein,
and the linker connects the domain to the catalytic domain of
the protein. In proteins having multiple lipoyl domains, such as
E. coli pyruvate dehydrogenase (PDH), which has three lipoyl
domains, the domains are arranged in tandem at the amino
terminus with linkers separating the domains from one another and from the catalytic domain (Fig. 1B). Biotinylated
proteins are the mirror image of the single lipoyl domain proteins; the modified domain is located at the carboxyl terminus,
and a proline plus alanine-rich sequence is found upstream of
the domain (Fig. 1A).
Acetyl-CoA carboxylase (ACC) catalyzes the first step in
fatty acid synthesis, the synthesis of malonyl-CoA from acetylCoA (12) (Fig. 2). ACC is a biotin-dependent enzyme, and like
1
The abbreviations used are: ACC, acetyl-CoA carboxylase; BCCP,
biotin carboxyl carrier protein; IPTG, isopropyl--D-thiogalactopyranoside; X-gal, 5-bromo-4-chloro-3-indoyl--D-galactoside; PDH, pyruvate
dehydrogenase.
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Biotin carboxyl carrier protein (BCCP) is the small
biotinylated subunit of Escherichia coli acetyl-CoA carboxylase, the enzyme that catalyzes the first committed
step of fatty acid synthesis. E. coli BCCP is a member of
a large family of protein domains modified by covalent
attachment of biotin. In most biotinylated proteins, the
biotin moiety is attached to a lysine residue located
about 35 residues from the carboxyl terminus of the
protein, which lies in the center of a strongly conserved
sequence that forms a tightly folded anti-parallel -barrel structure. Located upstream of the conserved biotinoyl domain sequence are proline/alanine-rich sequences of varying lengths, which have been proposed
to act as flexible linkers. In E. coli BCCP, this putative
linker extends for about 42 residues with over half of the
residues being proline or alanine. I report that deletion
of the 30 linker residues located adjacent to the biotinoyl domain resulted in a BCCP species that was defective in function in vivo, although it was efficiently biotinylated. Expression of this BCCP species failed to restore normal growth and fatty acid synthesis to a temperature-sensitive E. coli strain that lacks BCCP when
grown at nonpermissive temperatures. In contrast, replacement of the deleted BCCP linker with a linker derived from E. coli pyruvate dehydrogenase gave a chimeric BCCP species that had normal in vivo function.
Expression of BCCPs having deletions of various segments of the linker region of the chimeric protein
showed that some deletions of up to 24 residues had
significant or full biological activity, whereas others
had very weak or no activity. The inactive deletion proteins all lacked an APAAAAA sequence located adjacent
to the tightly folded biotinyl domain, whereas deletions
that removed only upstream linker sequences remained
active. Deletions within the linker of the wild type BCCP
protein also showed that the residues adjacent to the
tightly folded domain play an essential role in protein
function, although in this case some proteins with deletions within this region retained activity. Retention of
activity was due to fusion of the domain to upstream
sequences. These data provide new evidence for the
functional and structural similarities of biotinylated
and lipoylated proteins and strongly support a common
evolutionary origin of these enzyme subunits.
22521
E. coli BCCP
FIG. 1. Diagrams of the E. coli BCCP and PDH E2 proteins and
their linker sequences. A, BCCP with the domain shown as an oval,
the linker as the wavy line, and the biotin as . A putative interaction
domain at the N terminus is depicted by a rectangle. B, the PDH E2
subunit using the same depictions except that coenzyme is depicted by
␥. C, the sequences of the PDH E2 linkers; D, the residue substitutions
made in replacing the BCCP linker with that from PDH-1.
are essentially the same as those of synthetic linker peptides
(17, 23).
In this paper, I report that a large deletion of the putative
linker region of the E. coli BCCP subunit results in a protein of
extremely compromised ACC activity in vivo. However, biological activity was restored upon insertion of a sequence derived
from the first linker of the E. coli PDH E2 subunit, indicating
that the lipoyl and biotinoyl linker regions have overlapping
functions. Deletion and insertion analyses of this chimeric protein and of the native BCCP showed that only a restricted
portion of the linker was required for biological activity and
that many sequences fail to provide linker function.
EXPERIMENTAL PROCEDURES
Construction of an accB Gene Encoding a PDH Linker—The synthetic 133-bp linker was constructed in three stages due to its length.
First, a cassette encoding the N-terminal end of the linker was produced
by annealing the complementary oligonucleotides 1 and 2 (Table I) and
ligating this to pMTL22 (26) digested with the HindIII and NgoMIV
(the cassette was designed such that complementary ends would result
upon annealing). Likewise, a cassette encoding the C-terminal end of
the linker was produced by annealing oligonucleotides 3 and 4 and was
ligated to pK19 (27) digested with NgoMIV plus SmaI. Ligation to the
SmaI end resulted in construction of a BsiWI site. The plasmids encoding the N-terminal and C-terminal halves of the linkers were then
digested with NgoMIV and HindIII or BsiWI, respectively; the linkerencoding fragments were purified, mixed in equimolar ratios, and then
ligated together with the accB plasmid pCY326 (28) digested with
HindIII and BsiWI in a three partner ligation. The resulting plasmids
were sequenced and found to contain a 179-bp vector-derived sequence
inserted between the two linker-encoding fragments. This vector fragment insert (apparently due to a mutation at the NgoMIV end of the
C-terminal cassette) was removed by digesting the plasmid with
NgoMIV and BglII and inserting a cassette of annealed oligonucleotides
5 and 6 to give the desired linker sequence. Two other plasmids were
also derived by cassette mutagenesis. A cassette encoding the sequence
PAAAA was obtained by annealing oligonucleotides 7 and 8 and inserting this cassette into BglI-digested pCY326 (the cassette was designed
with complementary ends and included a new BstZI site and retained
the BglI site at one end of the inserted sequence). Finally, a cassette
designed to delete the EAPAAA sequence adjacent to the biotinoyl
domain of the wild type gene was obtained by annealing oligonucleotides 9 and 10 and ligating this product to pCY326 digested with NcoI
and BsiWI. This cassette introduced a BglII site and inactivated the
BsiWI site.
A series of deletions within the region encoding the chimeric linker
were made by restriction digestion followed by blunt end ligation done
at low DNA concentrations to favor recircularization. In most cases, one
or both of the ends resulting from digestion were either filled in or
resected by treatment with either phage T4 DNA polymerase plus the
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all biotin enzymes, the cofactor must be covalently attached to
ACC for enzyme activity. In E. coli and many other bacteria,
the functional enzyme consists of two copies of each of four
different subunits (13–15). The biotin cofactor is attached to
the biotin carboxyl carrier protein (BCCP) subunit, the sole
biotinylated protein of this organism (Fig. 2). In the first partial
reaction, the heterocyclic ring of the attached biotin moiety is
carboxylated by the biotin carboxylase subunit. The carboxyl
group is then transferred from biotin to acetyl-CoA to produce
malonyl-CoA by carboxyltransferase, a complex of two proteins
called ␣ and . Similar ACCs (referred to as heteromeric ACCs)
are found in the chloroplasts of many plants (16). The structure
of the biotinoyl domains of the E. coli acetyl-CoA subunit,
BCCP (also called AccB) (2, 4 – 6) and a related protein, the
Propionibacterium shermanii 1.3 S transcarboxylase subunit
(3), have been determined. In both cases, the structures of the
protein segments located upstream of the biotinoyl domain
could not be determined due to their high degree of mobility. No
structures could be detected in these protein segments by either x-ray diffraction or multidimensional heteronuclear NMR
spectroscopy.
Given the similarities between the biotinoyl and lipoyl domains, it seemed plausible that the putative biotinoyl domain
linker regions might play roles similar to those of the lipoyl
domain linkers. However, an argument against this notion is
that the sequences of biotinoyl and lipoyl linkers cannot be
aligned for more than a few residues (Fig. 1). Moreover, in the
linkers of some biotinoyl proteins, other small residues (generally serine or glycine) have replaced alanine, and the linkers of
lipoyl proteins are richer in charged residues than are the
biotinoyl linkers. On the other hand, given the conserved domain structures, it seemed unlikely that the conserved proline/
alanine-rich nature of the neighboring amino acid sequences
could be accidental. The E. coli PDH linkers (Fig. 2A) have been
extensively studied, and the available data indicate that the
combination of proline and alanine results in an extended, but
flexible, structure (1, 17–25). In the E. coli PDH complex, the
linker regions are known to be highly mobile and exposed to
solvent. The mobility of these linker regions was first detected
by NMR studies of the intact complex (20, 21). Despite the very
high molecular weight of the E. coli PDH complex (5–10 ⫻ 106),
a set of sharp resonances are observed that disappear with loss
of the lipoyl domains (from proteolytic cleavages within the
linkers). Moreover, the spectra observed in the intact complex
FIG. 2. The reaction mechanism of E. coli acetyl-CoA carboxylase. BCCP is encoded by the accB gene, whereas biotin carboxylase is
encoded by the accC gene (15). The two subunits involved in carboxyltransferase activity are encoded by the accA and accD genes (14). The
covalently bound biotin of BCCP carries the carboxylate moiety.
22522
E. coli BCCP
TABLE I
Oligonucleotides used in this work
Oligonucleotide
Sequence
1
2
3
4
5
6
7
8
9
10
11
12
5⬘-AGCTTACGACGGCGCAGCAGACGCTGCACCTGCGCAGGCAGAAGAGAAGAAAGAAGCAGCG
5⬘-CCGGCGCTGCTTCTTTCTTCTCTTCTGCCTGCGCAGGTGCAGCGTCTGCTGCGCCGTCGTA
5⬘-CCGGCTGCAGCACCAGCGGCTGCGGCGGCGGAGATCTCCGGTCACATCGTAC
5⬘-GTACGATGTGACCGGAGATCTCCGCCGCCGCAGCCGCTGGTGCTGCAGCCGG
5⬘-CCGGCTGCAGCACCAGCGGCTGCGGCGGCGGA
5⬘-GATCTCCGCCGCCGCAGCCGCTGGTGCTGCAG
5⬘-CGGCCGCTGCAGCTC
5⬘-CTGCAGCGGCCGGAG
5⬘-GAGATCTCCGGTCACATTGTAC
5⬘-AATGTGACCGGAGATCTCCATG
5⬘-CATGGAAGCGCCGGCAGCAGCGGAAATCAGTGGTCACATCGTAG
5⬘-GCGCCTACGATGTGACCACTGATTTCCGCTGCTGCCGGCGCTTC
plasmid was digested with NcoI (which cuts within accB) and KasI
(which cuts within the 1.3 S coding sequence), and a cassette made by
annealing the complementary oligonucleotides 11 and 12 was then
inserted by ligation.
Screening of Deleted Plasmids—The products of most of the deletion/
insertion ligations were transformed into strain CY1478, which carries
a chromosomal bioBFC-lacZ fusion and overproduces lacI repressor
from an F⬘ episome. Strain CY1478 was constructed by conjugational
mating of strain BM2661 (31) with NovaBlue (Novagen) with selection
for recombinants resistant to streptomycin and tetracycline to introduce the lacIQ episome. The transformants were tested on RB medium
containing 80 nM biotin and X-gal (40 g/ml), ampicillin, and tetracycline. Colonies were patched onto two plates of this medium, one supplemented with glucose (0.4% final concentration) and the other lacking
glucose. The plates were then incubated at 37 °C overnight. Colonies
that were very dark blue on the plate lacking glucose but pale blue on
the glucose-containing plates were in frame deletions, whereas colonies
that were white on both plates were out of frame fusions (the rationale
is explained under “Results”). The lacIQ episome was introduced to
decrease expression from the vector lac promoter, which is responsible
for BCCP expression in pCY326 and its derivatives. Glucose addition
was used to further decrease BCCP expression by decreasing cAMP-dependent expression from the lac promoter. Low level BCCP expression
was desired, because high level expression of this protein is toxic to
E. coli and also to allow colony scoring by use of X-gal. All constructs
were confirmed by DNA sequencing done by the Keck Genomics Center
of the University of Illinois. Treatment with T4 DNA polymerase (New
England Biolabs) was done in the restriction enzyme digestion buffer
supplemented with the four deoxynucleotide triphosphates each at 0.25
mM. One unit of polymerase was added, and the reaction was incubated
at 16 °C for 15–30 min followed by phenol treatment to inactivate the
polymerase. Treatment with mung bean nuclease (New England Biolabs) was done in the restriction enzyme digestion buffer supplemented
with 1 mM ZnSO4. Ten units of nuclease were added, followed by
incubation at 30 °C for 1 h and then phenol extraction to inactive the
nuclease.
Protein Expression—Each of the altered accB genes was cloned together with the downstream kanamycin resistance gene into the regulated expression vector pCY465 (28) as previously described.
Other Methods—Measurement of protein biotinylation by labeling
growing cultures with [8,9-3H]biotin, gel electrophoresis, fluorography,
and bacterial media were as described previously. A sample of BCCP-87
mixed with the full-length BCCP was made by IPTG induction of strain
TM126, a wild type strain carrying pTM52 (32) in [3H]biotin-containing
medium. Plasmid pTM52 was constructed by insertion of the BCCP-87
encoding NcoI-HindIII segment of pLS4 (15) into expression vector
pKK233-2 (33) cut with the same enzymes.
RESULTS
Essential Nature of the BCCP Linker—The determined
structures of the BCCP biotin domain begin at residues Gly77
and Ile78, although these residues and the next few residues
(up to Ile82) are considerably more mobile than the residues of
the tightly folded domain (2, 4 – 6). In preliminary work, the
accB gene was modified to introduce a BglII site that overlapped codons 77 and 78. The DNA segment lying between this
new site and the naturally occurring HindIII site was then
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four nucleoside triphosphates or by treatment with mung bean nuclease
in order to give the proper reading frame. The 3⬘ to 5⬘ exonuclease
activity of the polymerase was used to remove 3⬘-extensions, whereas
5⬘-extensions were removed by mung bean nuclease. For brevity, the
abbreviations T4 and MB following the restriction enzyme used to
generate the extension will be used to signify these manipulations (the
conditions used are given below). The constructions were as follows: ⌬1,
HindIII(T4) and PstI(T4); ⌬2, FspI and NaeI; ⌬4, FspI and BglII(T4);
⌬5, BstAPI(T4); ⌬6, HindIII(T4) and NgoMIV(T4); ⌬8, BsgI(T4) and
PstI(T4); ⌬9, NgoMIV(MB) and BglII(MB); ⌬10, NgoMIV(MB) and
PstI(T4); ⌬11, HindIII(MB) and FspI; ⌬12, HindIII(MB) and NaeI; ⌬13,
HindIII(T4) and BglII(T4). The ⌬15 plasmid resulted from the same
manipulations as ⌬10 but suffered a 27-bp deletion presumably via
illicit recombination in vivo.
A series of manipulations of the wild type BCCP linker region were
also made. The construct encoding the ⌬14 BCCP resulted from digestion of pCY326 with NcoI followed by MB treatment. The MB was
inactivated, and the DNA was digested with HindIII, treated with T4,
and ligated. The construct encoding the ⌬17 BCCP resulted from annealing the complementary oligonucleotides 9 and 10 and ligating the
cassette to pCY326 digested with NcoI plus BsiWI (the annealed cassette was designed such that complementary ends would result). The
⌬18 and ⌬19 BCCP constructs were made by insertion of antibiotic
cassettes into the introduced BglII site of the ⌬17 construct. The
BamHI tetracycline resistance-encoding fragment of p34s-Tet (29) was
ligated to the BglII-cut ⌬17 plasmid followed by selection for resistance
to both tetracycline and kanamycin. The tetracycline resistance determinant was then eliminated by SstI digestion and ligation. The construction of ⌬19 followed a similar route except that the SstI tetracycline resistance fragment p34s-Tet was flanked by two copies of the
BglII-SstI fragment of the multiple cloning site of pMTL25 (26). The
tetracycline determinant was then eliminated by MluI digestion followed by ligation to give the construct encoding ⌬19. Construction of the
genes encoding the ⌬20, ⌬21, ⌬22, and ⌬23 BCCPs used a 12-bp cassette
made by annealing the complementary oligonucleotides 7 and 8 followed by treatment with MB and phage T4 polynucleotide kinase plus
ATP. The blunt-ended cassette was then ligated to pCY326 digested
with NcoI plus BsiWI and treated with T4. DNA sequencing showed
three different products: insertion of one copy of the cassette in the two
possible orientations (⌬22 and ⌬22) and two copies inserted in a head to
tail configuration (⌬21). The gene encoding ⌬21 was then cut with PstI
and religated to give ⌬23. The genes encoding ⌬24 and ⌬27 were derived
from that encoding ⌬23 by insertion of the gentamycin resistance encoding the PstI fragment of p34s-Gm (29) into the PstI site of the ⌬23
construct. The resistance gene was then removed by digestion with
XbaI digestion to give ⌬25 or SstI digestion to give ⌬27. The gene
encoding ⌬26 resulted from insertion of the same oligonucleotide cassette as ⌬17 except that the sticky ends were removed by T4 treatment,
and the resulting blunt end cassette was ligated to pCY326 digested
with NcoI plus BsiWI and then treated with T4. The resulting recombinant plasmids were then screened for products with the cassette
inserted in the orientation opposite that of ⌬17. The gene encoding the
I28 insertion was constructed by annealing the complementary oligonucleotides 7 and 8 and ligating the resulting cassette to pCY326
digested with BglI. The gene encoding the BCCP-P. shermanii 1.3 S
chimeric protein was constructed from pCY325 (28) and p1.3t (30) by
ligating the SalI-BglII fragment of p1.3t into pMTL21 digested with
SalI plus BamHI. The resulting plasmid was digested with EcoRI and
BamHI and ligated to the EcoRI-BglII accB fragment of pCY325. This
E. coli BCCP
FIG. 3. Growth of strain CY1336 carrying plasmids encoding
wild type and mutant BCCPs. Early stationary phase cultures
(grown at 30 °C in the absence of IPTG except as noted) of strain
CY1336 (the temperature-sensitive G133S BCCP mutant) carrying
plasmids that encoded various mutant BCCPs were serially diluted in
10-fold steps, and 0.01-ml samples were spotted onto plates of RB
medium containing 75 M IPTG plus the antibiotics necessary for
plasmid maintenance, and the plates were incubated at 42 °C for 48 h
(A) or 24 h (B). The leftmost spot was the undiluted culture, the next spot
to the right was a 10-fold dilution, and successive 10-fold dilutions
proceeded to the right. The greater growth of ⌬13 in A is due to
pregrowth at 30 °C in the presence of 75 M IPTG. The DASMEP
protein is defective in biotinylation (10) and thus fails to complement
the accB temperature-sensitive mutant strain (28).
dehydrogenase (Fig. 1). This linker was chosen over the other
two E. coli PDH E2p linkers, since its length was exactly that
of the proline/alanine-rich region of BCCP, and thus no artificial truncation or extension was required to fit this linker into
BCCP. The restriction sites were included to simplify manipulations of the amino acid sequence. If insertion of the new
linker allowed biological function of the chimeric BCCP, then
construction of deletions that resulted in loss of function could
define the important parts of the linker. If, on the other hand,
the chimeric BCCP was nonfunctional, substitutions with segments of the BCCP sequence could be made in attempts to
restore function.
The striking result was that the chimeric BCCP was fully
functional at both 37 and 42 °C in vivo, indicating that the
proline/alanine-rich regions of BCCP and PDH linker 2 functioned in a similar manner. Given this result and the prior
studies demonstrating mobility and flexibility of the PDH linkers both in situ and as isolated peptides, it can be confidently
predicted that upon structural analysis, the proline/alaninerich region of BCCP shall be found to be a mobile linker.
Essential Segments of the Chimeric BCCP Linker Region—A
series of deletions were made within and upstream of the linker
region of the chimeric protein by use of the introduced restriction sites. These deletions were all made upstream of Gly80, the
first highly structured residue of the conserved domain (2,
4 – 6), since prior work had shown that deletion of residues at
the amino or carboxyl ends of biotinoyl domain sequences results in the loss of biotinylation due to loss of domain structure
(37, 38). Although BCCP residues 77–79 have defined secondary structures, these residues were considered candidates for
deletion or substitution, since the residues are extended from
(and do not interact with) the main body of the domain protein
and have been considered as part of the linker region (4). These
residues are highly mobile in the NMR analyses and have high
crystallographic B values and thus seemed reasonable candidates for deletion or substitution. Moreover, these residues are
not conserved in the BCCPs of Pseudomonas aeruginosa (39)
and Bacillus subtilis (40), both of which have been shown to
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deleted by restriction digestion followed by treatment with
phage T4 DNA polymerase plus the four deoxynucleotide
triphosphates and phage T4 DNA ligase. These manipulations
removed most of the putative linker region. It should be noted
that this construct would not be expected to produce an active
BCCP species, because the translation frame would be out-offrame relative to the accB coding sequence. However, such in
vitro filling and ligation reactions have a significant error rate
such that rare in-frame constructs were expected (34). In-frame
constructs were detected by use of the biotin operon regulatory
system (35–37). Overproduction of a biotin acceptor protein
results in increased expression of the biotin biosynthetic
operon due to consumption of biotinyl-AMP by protein biotinylation (37). This increase in transcription is readily detected by
use of a host strain (strain CY1478) carrying a promoter fusion
construct in which a biotin operon promoter drives expression
of E. coli -galactosidase (31). The increased biotin operon
expression is detected by increased cleavage of a chromogenic
-galactosidase substrate. In the present case, correctly processed constructs would be out-of-frame, and the lack of a
biotin domain expression would result in white colonies on
medium containing X-gal, whereas the rare in-frame constructs that accepted biotin (and thereby result in increased
biotin operon expression) would give blue colonies. This screen
was used in the construction of the deletions reported below,
and all blue colonies were shown by DNA sequencing to have
in-frame sequences (a few pale blue colonies were found to
encode out-of-frame constructs). In the case of the HindIIIBglII deletion, DNA sequencing showed an in-frame construct
(called ⌬13) that removed 30 residues of the putative BCCP
linker and resulted from incomplete filling of the BglII site.
The function of the ⌬13 construct and the other constructs in
this paper were tested in vivo as described previously (28). In
brief, the altered accB genes were inserted into an expression
plasmid that expressed the altered BCCP at a level comparable
with that from the wild type chromosomal accB gene. The
expression constructs where then transformed into E. coli accB
mutant strain, CY1336, which encodes a temperature-sensitive
BCCP (G133S BCCP) that is rapidly degraded upon shift to
37 °C (or higher temperatures). In the absence of expression of
a functional plasmid-encoded BCCP gene, strain CY1336 fails
to grow, whereas expression of a functional protein permits
growth (hence giving genetic complementation of the host accB
mutation). At 37 °C, growth was found to require about 8% of
the normal level of BCCP (28). Strain CY1336 expressing the
wild type gene grows well at 37 °C and appreciably more slowly
at 42 °C.
When the ⌬13 BCCP was expressed in strain CY1336, the
strain grew very poorly at 37 °C and failed to grow at 42 °C
(Fig. 3, A and B). As expected from prior work (28), the barely
detectable growth of the strain observed at 37 °C required
induction of gene expression with IPTG. Therefore, deletion of
the BCCP linker residues resulted in a protein that was virtually without activity in vivo.
Functional Replacement of the BCCP Linker with Linker 1 of
the E. coli PDH E2p Subunit—Two approaches were considered to dissect the role of the linker region in BCCP function.
The first approach was a conventional deletion analysis, but
this was inconvenient due to a scarcity of restriction sites in the
sequence encoding the linker. A second approach was to substitute a known linker sequence for that of BCCP and thereby
obtain a well studied linker. This second approach was taken
and the coding sequence of the chosen linker was redesigned to
include useful restriction sites. In this construct, 28 residues of
the putative linker of BCCP were replaced with the sequence
that links the outermost two lipoyl domains of E. coli pyruvate
22523
22524
E. coli BCCP
fully replace the function of E. coli BCCP in vivo (the latter
protein also has a one-residue deletion within this segment).
Finally, none of the BCCP residues upstream of residue 81
show any structure-based alignment with the other biotinyl
domain of known structure, the P. shermanii 1.3 S trancarboxylase subunit (28), indicating that these residues do not
play a role in biotin domain structure.
Upon construction and testing in vivo function of a number of
constructs (Figs. 3 and 4 and Table II), BCCP function was
found to tolerate rather large deletions, whereas some small
deletions gave nonfunctional or poorly functional BCCPs. For
example, expression of the ⌬1 and ⌬6 BCCPs, containing deletions of 23 and 19 residues, respectively, resulted in normal
growth of CY1336 at 37 and 42 °C, whereas ⌬15, a smaller
deletion of only nine residues, completely failed to support
growth. The deletions (⌬4, ⌬13, and ⌬15) that were the most
defective in supporting growth of CY1336 lacked the sequence
of four consecutive alanine residues located just upstream of
the biotinoyl domain. This region seemed very sensitive to
sequence alterations, since deletions that fused the biotinoyl
domain to an AAP sequence (⌬15), an AAPA sequence (⌬4), or
a single alanine (⌬13) were all inactive. In contrast, deletions of
upstream proline/alanine sequences as in the ⌬1, ⌬2, ⌬5, ⌬6,
⌬11, and ⌬12 BCCPs had little or no effect on growth. The only
exception to this picture was ⌬8, a deletion of 29 residues.
Although in ⌬8 the sequences just upstream of the biotinoyl
domain were left intact, expression of the mutant BCCP resulted in only slow growth of CY1336 at 37 °C. However, interpretation of this result is not straightforward, since expression
of the ⌬8 BCCP also allowed growth at 42 °C (all other constructs that grew poorly at 37 °C failed to grow at the higher
temperature). Note that an observed lack of activity in the
biological test system could be due to degradation of the altered
proteins such as endoproteolytic cleavage of the altered linker
sequences. To test this possibility, each of the constructs was
expressed in strain CY1336 at a nonpermissive temperature
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FIG. 4. Analyses of protein biotinylation in strains expressing
wild type and mutant BCCPs. All analyses were of cultures grown at
30 °C in the presence of IPTG and then shifted to 38 °C for 70 min. The
panels shown are SDS-polyacrylamide gel analyses of [3H]biotin-labeled proteins. Cells from 0.2 ml of [3H]biotin-labeled cultures were
boiled in SDS buffer and applied to SDS 16% polyacrylamide gels as
described previously (28). Fluorograms of the gels are shown. The
samples loaded are given above each lane (see Table II for sequences).
A sample of [3H]biotin-labeled BCCP-87 (43) made by induction of
strain TM126 (see “Experimental Procedures”) was included as a
marker for possible degradation products. The gel was submitted to a
prolonged exposure in order to show the traces of chromosomally encoded G133S BCCP that provide an internal mobility standard. (The
lines in the gel are cracks resulting from the drying process.)
(so the chromosomally encoded G133S protein would be degraded) and labeled with [3H]biotin, and the radioactive proteins were analyzed by SDS-gel electrophoresis. Expression of
each of the deleted BCCPs at 38 °C in the presence of [3H]biotin
gave a radioactive band of the expected electrophoretic mobility. Fig. 4 shows some of these data. Two of the constructs (⌬1
and ⌬2, data not shown) showed minor amounts of smaller
biotinylated proteins in addition to the full-length proteins
consistent with some endoproteolytic processing. However, the
strains expressing these proteins grew normally, and thus the
loss of protein due to processing was of no consequence.
Analysis of the Native BCCP Linker Region—Deletion and
insertion studies were also done on the native BCCP linker.
First, based on the results obtained with the chimeric protein,
⌬14, a deletion similar to ⌬13, but retaining the proline/alanine-rich sequence adjacent to the biotinoyl domain, was constructed. Expression of the ⌬14 protein gave only modestly
reduced growth, and thus restoration of only 7 of the 30 residues deleted in ⌬13 resulted in return of most of the biological
activity. Of these seven residues, it seemed likely (given the
above results) that the APAAA sequence would play an important role in this restoration of biological activity. Therefore,
this sequence was deleted from the native linker to give the ⌬17
protein with the anticipation that this protein should show
decreased biological activity. However, expression of the ⌬17
protein gave essentially wild type growth (Fig. 3). Hence, the
presence or absence of APAAA sequences adjacent to the biotinoyl domain seemed to give apparently conflicting results in
⌬17 BCCP versus ⌬13 BCCP. This apparent conflict could be
explained if in ⌬17 BCCP upstream proline/alanine-rich sequences could functionally substitute for the deleted residues,
whereas in ⌬13 no sequences were available to act as surrogates of the deleted residues. To test whether upstream residues were responsible for function of the ⌬17 BCCP, two arbitrary sequences were inserted adjacent to the biotinoyl domain
of the ⌬17 protein to act as possible spacers or insulators
between the domain and the upstream proline/alanine-rich
sequences. The sequences used were those encoded by symmetrical multiple cloning sites from two different cloning vectors,
and these were inserted immediately upstream of the biotinoyl
domain of ⌬17 to give the ⌬18 and ⌬19 BCCPs. Although
expression of the ⌬18 protein allowed growth at 37 °C, growth
at 42 °C was greatly decreased. Upon expression of the ⌬19
protein, growth at 37 °C was almost normal, and growth also
proceeded at 42 °C, albeit at a rate about 50 – 60% of that seen
upon expression of the wild type protein. Therefore, the sequence IPGYRARYPG of ⌬18 acts as an effective barrier between the biotin domain and the upstream sequences. Note
that the proteins with these altered linkers were well expressed and stable in vivo (Fig. 4).
Other BCCP Constructs—A number of other alterations of
the wild type BCCP sequence were made (data not shown).
Several encoded proteins were unstable or poorly biotinylated.
However, four constructs are worthy of note. Substitution of
the EAPAAAEISGHI sequence with NVTGDL (⌬26), LQRPRPLQ (⌬21), LQVDSRVDLQ (⌬25), or NVTGDPRVPSSVPGDL
(⌬20) gave proteins that were stable and well biotinylated.
However, upon expression, none of the three proteins supported growth of strain CY1336. Therefore, residues 77– 82 of
BCCP can be substituted with very different residues without
major effects on the structure of the biotinoyl domain as assayed by the sensitive assays of biotinylation and stability to
proteolysis in vivo (36), and thus the lack of biological activity
must be attributed to the alterations of the linker. Finally, a
more ambitious chimeric protein was made in which the BCCP
biotinoyl domain was replaced with the only other biotinoyl
22525
E. coli BCCP
TABLE II
Growth resulting from expression of deleted and inserted BCCP species
Deleted residues are denoted by the empty spaces and insertions by lines (/\). The sequences above the line are the BCCP/PDH chimeric linker
(designated as PDH) and its deletion variants. The sequences below the line are the BCCP linker and its deletion variants. The first and last BCCP
residues shown are Pro37 and Val83, respectively. Growth is denoted by colony size at the temperature given on medium containing 75 M IPTG
(28), with pp signifying pinpoint colonies visible to the eye, (pp) signifying pinpoint colonies visible only with a magnifying lens, and O signifying
no detectable growth. Before testing the cultures were grown at 30 °C in the absence of IPTG.
Downloaded from http://www.jbc.org/ by guest on April 24, 2020
domain of known structure, that of P. shermanii transcarboxylase (3). The fusion junction between the two proteins was a
structurally conserved glycine residue such that the last residue of the BCCP linker, Gly80, was replaced with Gly48 of the
transcarboxylase 1.3 S subunit. Upstream of this glycine residue were the N-terminal 79 residues of BCCP, and downstream
were the 75 C-terminal residues of the 1.3 S subunit biotinoyl
domain. Expression of this chimeric protein failed to allow any
detectable growth of mutant strain CY1336 at any nonpermissive temperature, although the chimeric protein was stable and
efficiently biotinylated. Therefore, despite the conserved do-
main structure, the biotinoyl domain of BCCP could not be
functionally replaced with a foreign biotinoyl domain.
DISCUSSION
The proline/alanine-rich region of BCCP is essential for function of the protein as shown by the deletion analyses reported.
The finding that a linker region from the E2 subunit of the
PDH complex functionally substitutes for the natural region
indicates that these protein segments play similar roles in the
two enzyme complexes, although the cognate enzymes have
markedly differing structures and catalytic activities. Since the
22526
E. coli BCCP
Study of the native BCCP linker gave a more complex picture
in that deletion of the proline/alanine sequence adjacent to the
domain had no effect on protein function. This lack of effect
appears due to functional replacement of these residues with
upstream proline/alanine sequences because introduction of
arbitrary sequences adjacent to the domain blocked BCCP
function. Therefore, the PDH linker seems to contain sequences that restrict the use of upstream residues as surrogates of those adjacent to the domain, whereas the native
BCCP linker lacks this property. Since mobile linker and loop
regions cannot be determined by the currently available techniques, the structures of such protein segments is a major
unsolved problem of structural biology. In hopes of gaining
structural insight, each of the linker sequences in this paper
was submitted to the PSIPRED version 2.2 protein structure
prediction program (available on the World Wide Web at bioinf.
cs.ucl.ac.uk/psipred/). This program is considered to be the
current state of the art for structure prediction from primary
sequence (about 80% accurate). (Note that PSIPRED prediction
of the BCCP biotinyl domain showed a highly accurate correspondence with the known domain structure, thus engendering
confidence in the program.) None of the linker residue alterations reported in this paper changed the predicted pattern of
secondary structure elements in wild type BCCP. The only
changes seen were in the lengths of the linker regions (predicted as coil by the program). Thus, there are currently no data
predicting which sequence or sequences can account for the
differing “insulating” properties of the PDH and BCCP linkers.
It would have been advantageous to supplement the genetic
complementation data presented in this paper with assays of
ACC activity. However, the ACC of E. coli readily dissociates
upon cell lysis, and hence the overall ACC activity cannot be
measured in crude cell extracts (12, 13, 42).
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structures of the biotinoyl and lipoyl domains can be largely
superimposed, it seems that not only the domain structures,
but also the structure of the linkers that connect these domains
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Interchangeable Enzyme Modules: FUNCTIONAL REPLACEMENT OF THE
ESSENTIAL LINKER OF THE BIOTINYLATED SUBUNIT OF ACETYL-CoA
CARBOXYLASE WITH A LINKER FROM THE LIPOYLATED SUBUNIT OF
PYRUVATE DEHYDROGENASE
John E. Cronan, Jr.
J. Biol. Chem. 2002, 277:22520-22527.
doi: 10.1074/jbc.M201249200 originally published online April 15, 2002
Access the most updated version of this article at doi: 10.1074/jbc.M201249200
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