Membrane budding driven by intra-cellular ESCRT-III filaments

TL;DR

This paper models the membrane budding process driven by ESCRT-III filaments using a continuum energy functional, revealing membrane adhesion as the key driver and highlighting the stabilizing role of ESCRT-III.

Contribution

It introduces a continuum energy model for ESCRT-III driven budding, emphasizing membrane adhesion over filament buckling as the main force.

Findings

Membrane adhesion drives budding, not filament buckling.

ESCRT-III stabilizes the bud after formation.

Non-equilibrium processes are required for membrane scission.

Abstract

Exocytosis is a common transport mechanism via which cells transport out non-essential macro-molecules (cargo) into the extra cellular space. ESCRT-III proteins are known to help in this. They polymerize into a conical spring like structure and help deform the cell membrane locally into a bud which wrapps the outgoing cargo. we model this process using a continuum energy functional. It consists of elastic energies of the membrane and the semi-rigid ESCRT-III filament, favorable adhesion energy between the cargo and the membrane, and affinity among the ESCRT-III filaments. We take the free energy minimization route to identify the sequence of composite structures which form during the process. We show that membrane adhesion of the cargo is the driving force for this budding process and not the buckling of ESCRT-III filaments from flat spiral to conical spring shape. However ESCRT-III stabilizes the bud once it forms. Further we conclude that a non-equilibrium process is needed to pinch off/separate the stable bud (containing the cargo) from the cell body.