Entropic Tension in Crowded Membranes

TL;DR

This paper demonstrates that crowding of membrane proteins induces an entropic tension that significantly influences the conformational transitions and function of membrane proteins, especially mechanosensitive channels, in biological membranes.

Contribution

It introduces a statistical mechanics model showing how crowding causes entropic tension affecting membrane protein gating, a novel insight into membrane biophysics.

Findings

Crowding induces an entropic tension in membranes.

This tension can alter gating energies by over 2 k_BT.

Crowding effects are significant under physiological conditions.

Abstract

Unlike their model membrane counterparts, biological membranes are richly decorated with a heterogeneous assembly of membrane proteins. These proteins are so tightly packed that their excluded area interactions can alter the free energy landscape controlling the conformational transitions suffered by such proteins. For membrane channels, this effect can alter the critical membrane tension at which they undergo a transition from a closed to an open state, and therefore influence protein function \emph{in vivo}. Despite their obvious importance, crowding phenomena in membranes are much less well studied than in the cytoplasm. Using statistical mechanics results for hard disk liquids, we show that crowding induces an entropic tension in the membrane, which influences transitions that alter the projected area and circumference of a membrane protein. As a specific case study in this effect, we consider the impact of crowding on the gating properties of bacterial mechanosensitive membrane channels, which are thought to confer osmoprotection when these cells are subjected to osmotic shock. We find that crowding can alter the gating energies by more than $2\;k_BT$ in physiological conditions, a substantial fraction of the total gating energies in some cases. Given the ubiquity of membrane crowding, the nonspecific nature of excluded volume interactions, and the fact that the function of many membrane proteins involve significant conformational changes, this specific case study highlights a general aspect in the function of membrane proteins.