Binding of anisotropic curvature-inducing proteins onto membrane tubes

TL;DR

This paper develops a mean-field theoretical model to understand how anisotropic curvature-inducing proteins bind and orient on membrane tubes, revealing phase transitions and effects of external forces on protein binding and membrane shape.

Contribution

It introduces a mean-field theory incorporating orientation-dependent excluded volume to describe protein binding and ordering on membrane tubes, including the effects of external forces.

Findings

Proteins undergo nematic transitions depending on density and tube radius.

External force induces a first-order transition in tube radius and protein orientation.

The theory matches membrane simulation results for short proteins, with deviations for long proteins.

Abstract

Bin/Amphiphysin/Rvs superfamily proteins and other curvature-inducing proteins have anisotropic shapes and anisotropically bend biomembrane. Here, we report how the anisotropic proteins bind the membrane tube and are orientationally ordered using mean-field theory including an orientation-dependent excluded volume. The proteins exhibit a second-order or first-order nematic transition with increasing protein density depending on the radius of the membrane tube. The tube curvatures for the maximum protein binding and orientational order are different and varied by the protein density and rigidity. As the external force along the tube axis increases, a first-order transition from a large tube radius with low protein density to a small radius with high density occurs once, and subsequently, the protein orientation tilts to the tube-axis direction. When an isotropic bending energy is used for the proteins with an elliptic shape, the force-dependence curves become symmetric and the first-order transition occurs twice. This theory quantitatively reproduces the results of meshless membrane simulation for short proteins, whereas deviations are seen for long proteins owing to the formation of protein clusters.