Photoswitching of model ion channels in lipid bilayers.

Juergen Pfeffermann,Christof Hannesschlaeger,Timur R Galimzyanov,Barbara Eicher,Sergey A Akimov,Peter Pohl,Georg Pabst,Toma N Glasnov,Danila Boytsov

doi:10.1016/j.jphotobiol.2021.112320

Abstract

Membrane proteins can be regulated by alterations in material properties intrinsic to the hosting lipid bilayer. Here, we investigated whether the reversible photoisomerization of bilayer-embedded diacylglycerols (OptoDArG) with two azobenzene-containing acyl chains may trigger such regulatory events. We observed an augmented open probability of the mechanosensitive model channel gramicidin A (gA) upon photoisomerizing OptoDArG's acyl chains from trans to cis: integral planar bilayer conductance brought forth by hundreds of simultaneously conducting gA dimers increased by typically >50% - in good agreement with the observed increase in single-channel lifetime. Further, (i) increments in the electrical capacitance of planar lipid bilayers and protrusion length of aspirated giant unilamellar vesicles into suction pipettes, as well as (ii) changes of small-angle X-ray scattering of multilamellar vesicles indicated that spontaneous curvature, hydrophobic thickness, and bending elasticity decreased upon switching from trans- to cis-OptoDArG. Our bilayer elasticity model for gA supports the causal relationship between changes in gA activity and bilayer material properties upon photoisomerization. Thus, we conclude that photolipids are deployable for converting bilayers of potentially diverse origins into light-gated actuators for mechanosensitive proteins.

Highlights

Key to the mechanical sensitivity of a membrane protein is the hy drophobic coupling between its hydrophobic transmembrane domains and the embedding bilayer's hydrophobic core [1,2]
Earlier approaches at character izing ΔGdef were focused on the interaction between depth-dependent changes in protein cross sectional area, A(z), with the lateral pressure profile intrinsic to lipid bilayers [9]; in that framework, an increase in lateral pressure within the acyl chain region mediated by, e.g., a change in lipid composition can be thought of as promoting a reduction in A(z) there, i.e., a structural change that might be of functional consequence
We studied the implications of photoisomerization of a membrane-embedded photolipid termed OptoDArG (Fig. 1) on the ac tivity profile of the mecha noresponsive (MS) model ion channel gramicidin A

Summary

Introduction

Key to the mechanical sensitivity of a membrane protein is the hy drophobic coupling between its hydrophobic transmembrane domains and the embedding bilayer's hydrophobic core [1,2]. It was demonstrated that the “lateral pressure model” for membrane protein regulation by Cantor [11] can be equivalently expressed using the theory of elasticity of lipid bilayers introduced by Helfrich [12,13]. The latter approach of quanti fying ΔGdef using a set of experimentally-accessible bilayer material properties – e.g. thickness, intrinsic curvatures of the constituent lipids, compressibility and bending modulus, B, membrane tension – has pre vailed [14]. The high-threshold tension-gated MscL is exquisitely modulated by alterations in mem brane hydrophobic thickness, dhc, [19] and spontaneous monolayer curvature, J0, as demonstrated by the asymmetric incorporation of lysophosphatidylcholine (LPC) [20]

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Conclusion