A network of chaperones prevents and detects failures in membrane protein lipid bilayer integration

João P L Coelho,Matthias Stahl,Nicolas Bloemeke,Kevin Meighen-Berger,Stephan A Sieber,Matthias J Feige,Carlos Piedrafita Alvira,Zai-Rong Zhang

doi:10.1038/s41467-019-08632-0

Abstract

A fundamental step in membrane protein biogenesis is their integration into the lipid bilayer with a defined orientation of each transmembrane segment. Despite this, it remains unclear how cells detect and handle failures in this process. Here we show that single point mutations in the membrane protein connexin 32 (Cx32), which cause Charcot-Marie-Tooth disease, can cause failures in membrane integration. This leads to Cx32 transport defects and rapid degradation. Our data show that multiple chaperones detect and remedy this aberrant behavior: the ER–membrane complex (EMC) aids in membrane integration of low-hydrophobicity transmembrane segments. If they fail to integrate, these are recognized by the ER–lumenal chaperone BiP. Ultimately, the E3 ligase gp78 ubiquitinates Cx32 proteins, targeting them for degradation. Thus, cells use a coordinated system of chaperones for the complex task of membrane protein biogenesis, which can be compromised by single point mutations, causing human disease.

Highlights

A critical step in the biosynthesis of membrane proteins is their correct topogenesis, i.e., the integration of each TM helix in the adequate orientation into the lipid bilayer[1]
Individual TM helices may fail to be integrated into the membrane either due to mutations[18] or due to structural/ functional requirements that compete with TM segment hydrophobicity[17,42]
We investigated the effects of incorrect topology formation/membrane integration on membrane protein quality control

Summary

Introduction

A critical step in the biosynthesis of membrane proteins is their correct topogenesis, i.e., the integration of each TM helix in the adequate orientation into the lipid bilayer[1]. Functional requirements very often compete with the structural stability of proteins[4], and membrane proteins are no exception to this rule: the hydrophobic interior of the lipid bilayer would energetically favor integration of nonpolar residues, but defined intra or intermolecular interactions of TM helices as well as functional requirements, e.g., providing a hydrophilic cavity in ion channels, can compromise membrane protein integration[5]. Membrane protein topogenesis and TM helix integration are intimately linked to the correct assembly of individual TM helices within the membrane[15,16]. It is likely that the cell has developed means to identify and either correct or dispose of incorrectly integrated TM proteins These mechanisms remain mostly unknown, but they are relevant since polar residues due to mutations in TM regions are very often associated with human disease[18,19]. Using Cx32 as a biomedically relevant model system, within this study we addressed the question if and how the ER quality control system can prevent, recognize and handle aberrant membrane protein lipid bilayer integration and topogenesis

Methods

Results

Conclusion