The molecular size of the extra-membrane domain influences the diffusion of the GPI-anchored VSG on the trypanosome plasma membrane.

Andreas J W Hartel,Markus Engstler,Susanne F Fenz,Marius Glogger,Nicola G Jones,Gernot Guigas,Matthias Weiss

doi:10.1038/srep10394

Abstract

A plethora of proteins undergo random and passive diffusion in biological membranes. While the contribution of the membrane-embedded domain to diffusion is well established, the potential impact of the extra-membrane protein part has been largely neglected. Here, we show that the molecular length influences the diffusion coefficient of GPI-anchored proteins: smaller proteins diffuse faster than larger ones. The distinct diffusion properties of differently sized membrane proteins are biologically relevant. The variant surface glycoprotein (VSG) of African trypanosomes, for example, is sized for an effective diffusion-driven randomization on the cell surface, a process that is essential for parasite virulence. We propose that the molecular sizes of proteins dominating the cell surfaces of other eukaryotic pathogens may also be related to diffusion-limited functions.

Highlights

Today, there is a growing body of evidence that additional parameters such as molecular crowding and protein size should be taken into account
The cells were transiently immobilised in a custom-made observation chamber, and diffusion coefficient and mobile fraction of the control variant surface glycoprotein (VSG) were determined by Fluorescence recovery after photo-bleaching (FRAP) analysis (Table 1)
The VSG size was increased by addition of monovalent streptavidin[21]

Summary

Introduction

There is a growing body of evidence that additional parameters such as molecular crowding and protein size should be taken into account. There is a strong selective pressure on the parasite VSG repertoire: VSGs have to be sufficiently similar to maintain the shielding function on the cell surface, e.g. during antigen switching, and they have to be sufficiently different to provide the molecular basis for antigenic variation[18]. This unusual homogeneity of the trypanosome surface coat makes the VSG an ideal tool for studies on the impact of the protein’s soluble domain on lateral diffusion. We ask two questions: (A) to what extent does the axis length of the VSG influence its diffusion properties and (B) do VSG dimensions correlate with potentially diffusion-limited biological functions ? We decided to tackle these questions in a sequential experimental approach that involves stepwise abstraction from the natural state

Methods

Results

Conclusion