Channels
ISSN: 1933-6950 (Print) 1933-6969 (Online) Journal homepage: https://www.tandfonline.com/loi/kchl20
Toward a structural blueprint for bilayer-mediated
channel mechanosensitivity
Navid Bavi, Charles D. Cox, Eduardo Perozo & Boris Martinac
To cite this article: Navid Bavi, Charles D. Cox, Eduardo Perozo & Boris Martinac (2017) Toward
a structural blueprint for bilayer-mediated channel mechanosensitivity, Channels, 11:2, 91-93, DOI:
10.1080/19336950.2016.1224624
To link to this article: https://doi.org/10.1080/19336950.2016.1224624
© 2017 Taylor & Francis
Accepted author version posted online: 17
Aug 2016.
Published online: 02 Sep 2016.
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CHANNELS
2017, VOL. 11, NO. 2, 91–93
http://dx.doi.org/10.1080/19336950.2016.1224624
AUTOCOMMENTARY
Toward a structural blueprint for bilayer-mediated channel mechanosensitivity
Navid Bavia,b, Charles D. Coxa,b, Eduardo Perozoc, and Boris Martinaca,b
a
Molecular Cardiology and Biophysics Division, Victor Chang Cardiac Research Institute, Darlinghurst, NSW, Australia; bSt Vincent’s Clinical
School, Faculty of Medicine, University of New South Wales, Darlinghurst, NSW, Australia; cDepartment of Biochemistry and Molecular Biology,
Institute for Biophysical Dynamics, University of Chicago, Chicago, IL, USA
ARTICLE HISTORY Received 11 August 2016; Accepted 12 August 2016
KEYWORDS EPR spectroscopy; force-from-lipid; K2P channel; lipid-protein interaction; Mechanosensitive channel; MscL; MscS; Pressure profile
Bacterial mechanosensitive (MS) channels serve as
excellent model systems to study the basic mechanisms underlying bilayer-mediated channel mechanosensitivity. There is extensive evidence showing that
these channels are gated by membrane tension according to the force-from-lipids concept.1 Increasing interest in this paradigm has arisen due to its applicability
to eukaryotic channels, in particular mechanicallygated 2-pore domain potassium (K2P) channels and in
all likelihood Piezo1. In its simplest form the principle
states that the force necessary for channel gating is
transmitted directly via the surrounding lipid bilayer.
However, while finding a mechanistic explanation to
this process is fundamental to our understanding of
mechanosensory transduction, the details of how this
occurs in individual MS channel families is still the
subject of intense debate.
Recently,2 we looked at the role of the amphipathic
N-terminal helix in the gating cycle of the bacterial
channel MscL. Although this helix was not resolved in
the first crystal structure of MtMscL, its functional
role was the subject of much speculation. Initially, it
was proposed to form a helical bundle representing a
second channel gate.3 Later on, the N-terminus was
resolved and shown to sit close to the solvent-lipid
interface. Blount and co-workers then provided evidence using multiple techniques that the N-terminus
functions as an ‘anchor’ during channel gating.4
Building on this work, we used electron paramagnetic resonance (EPR) spectroscopy combined with a
multi-scale computational approach and single-channel recording showing that this helix acts as an integral force-bearing element during activation gating. It
mechanically couples bilayer deformation to pore
expansion via a glycine hinge at the inner side of the
pore-lining helix.
Our data unequivocally confirms the localization of
the N-terminal helix at the solvent-lipid interface, a
fact further solidified by a crystallographic structure of
an archaeal MscL homolog. Interestingly, not only
does the N-terminus drive the tilting of the pore
-lining helix, but it seems to also drive radial pore
expansion. For this to occur, lipids protrude into intersubunit cavities and essentially wrap around the upper
portions of the N-termini2; so that during gating the
lipid has to be largely stripped from these cavities to
enable expansion. However, MD simulation shows that
while lipids do move from these cavities, they still
strongly interact with a number of the N-terminal residues (Fig. 1). This suggests that lipid movement is
actively ‘dragging’ the N-terminus and driving gating.
Lipid-filled pockets equivalent to those identified in
MscL are also present in other mechanically-gated
channels such as MscS5 and K2P channels. Indeed,
lipid seemed to have such a central position in the initial TRAAK structure, that it was suggested to act as a
gate, an unlikely possibility. However, lipid-filled protein pockets are also found in the structures of many
non-mechanosensitive membrane-embedded proteins, including the K2P channel TWIK-16 and the
CONTACT Boris Martinac
b.martinac@victorchang.edu.au
Victor Chang Cardiac Research Institute, Lowy Packer Building, 405 Liverpool St., Darlinghurst, NSW 2010, Australia.
Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/kchl.
Autocommentary to: Bavi N, Cortes DM, Cox CD, Rohde PR, Liu W, Deitmer JW, Bavi O, Hill AP, Rees D, Corry B, et al. The role of MscL amphipathic N terminus indicates a blueprint for bilayer-mediated gating of mechanosensitive channels. Nat Commun. 2016 Jun 22;7:11984; PMID: 27329693; http://dx.doi.org10.1038/
ncomms11984
© 2017 Taylor & Francis
92
N. BAVI ET AL.
Figure 1. (A) single MscL subunit in the closed (left) and open (right) state. Each structure has the interaction energy with lipid molecules mapped onto it and the corresponding pressure-profile over-layed. Beneath shows the potential mechanisms that may enable
channel mechanosensitivity, either dragging of structural elements or elastic strain energy induced gating (a la jack-in-the-box).
voltage-dependent channel Kv1.2. What then is the
mechanistic relevance of these lipid-filled protein
pockets in mechanically activated channels?
In the case of MscS, the proposal is that as tension
is applied, lipids move from these cavities enabling
gating.5 In this scenario, gating is presumably driven
by the stored strain or imposed elastic energy caused
by the repulsive forces of the acyl chains. Malcolm
et al., have also suggested a similar pressure-profile
mediated mechanism.7 Membrane embedded proteins
are subject to large anisotropic forces in the transverse
direction,8 and as we have shown, there is a bilateral
relationship between the bilayer pressure-profile and
integral membrane proteins (Fig. 1).2
In fact, the cavity between TM2 and TM3 in MscS
(Fig. 1) houses a number of functionally critical residues (F68, L111, L115).9 If a model using stored elastic
energy were dominant, mutations to these residues
should result in a gain-of-function phenotype associated with the lower degree of lipid protrusion.
However, hydrophilic substitutions of these residues
(i.e. F68S) results in a loss of mechanosensitivity.9
Given the presence of lipid tails in this region5 it is
more likely that, similar to MscL, some lipid-mediated
dragging is necessary for channel gating.
In order to address such questions, we need to look
at both the transbilayer pressure-profile (in the presence of protein) and the interactions between the protein and its annular lipids. While our data clearly
indicates a “dragging” scenario in MscL, in other cases
(e.g., MscS), there may well be a balance between these
2 mechanisms, so that the stored strain energy
imposed by the repulsive forces in the acyl chains may
act in concert with direct “dragging” of protein
elements.
It should also be pointed out that a dragging force
applied to an integral membrane protein need not
simply cause radial motion. For example, the rotational motion of the TM1/TM2 paddle of MscS under
applied force does not preclude “dragging” as its
CHANNELS
ultimate movement depends on the mechanical properties of the protein and the points at which the protein is anchored to the membrane.
If a “dragging” mechanism is, at least in part,
responsible for the mechanosensitivity of MS channels, we would expect to see these horizontal forcecoupling helices in other ion channel families, also
juxtaposed to the pore-lining helices. Indeed, this is
the case at least in K2P channels, where the C-terminal
helix seems to be important in mechanical-driven gating. Furthermore, similar structures may play a
mechanosensitive role in Piezo and TRP channels
(although we should note that the force-from-lipids
paradigm is yet to be conclusively shown to apply to
any TRP channel). Importantly, the role of these horizontal force-coupling helices should mostly depend
on their absorption or adsorption to the bilayer and as
such, there is no expectation of high sequence similarity. The only requirement is that they are directly
linked to pore-forming helices. Thus, horizontal
force-coupling helices may represent an important
conserved structural entity that underlies channel
mechanosensitivity.
Disclosure of potential conflicts of interest
No potential conflicts of interest were disclosed.
Funding
N.B. has been supported by a University International Postgraduate Award (UIPA) from the University of New South
Wales. This project was supported by an Australian Research
Council grant (DP160103993) and a Principal Research Fellowship to B.M. from the National Health and Medical
Research Council of Australia. This work was also supported
in part by funds from the Office of Health and Medical
Research, NSW State Government.
93
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