Membrane protein insertion: mixing eukaryotic and prokaryotic concepts

Abstract

Proteins are translocated across or inserted into membranes by machines that are composed of soluble and membrane-anchored subunits. The molecular action of these machines and their evolutionary origin are at present the focus of intense research. For instance, our understanding of the mode of insertion of beta-barrel membrane proteins into the outer membrane of endosymbiotically derived organelles has increased rapidly during the past few years. In particular, the identification of the Omp85/YaeT-involving pathways in Neisseria meningitidis, Escherichia coli and cyanobacteria, and homologues of Omp85/YaeT in chloroplasts and mitochondria, has provided new clues about the ancestral beta-barrel protein insertion pathway. This review focuses on recent advances in the elucidation of the evolutionarily conserved concepts that underlie the translocation and insertion of beta-barrel membrane proteins.

Figure 1

β-barrel outer membrane protein insertion. The translocation system for β-barrel outer membrane proteins (OMPs) in (A) bacteria, (B) mitochondria and (C) chloroplasts are shown. Arrows indicate the translocation pathway. Please note, the stoichiometry of the bacterial system is not experimentally supported. In the case of chloroplasts, the translocation path is dissected into that established for Toc75 and possible routes for other OMPs. The question marks indicate the current state of knowledge. IM, inner membrane; IMS, intermembrane space; LPS, lipopolysaccharide; OM, outer membrane; PG, peptidoglycan layer; PL, phospholipid; PM, plasma membrane; PP, periplasm.