An ER translocon for multi-pass membrane protein biogenesis.

Philip T Mcgilvray,S Andrei Anghel,Michael J Trnka,Alma L Burlingame,Frank Zhong,Arunkumar Sundaram,James R Fuller,Hong Hu,Robert J Keenan

doi:10.7554/elife.56889

Philip T Mcgilvray, S Andrei Anghel + Show 7 more

Open Access

https://doi.org/10.7554/elife.56889

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Abstract

Membrane proteins with multiple transmembrane domains play critical roles in cell physiology, but little is known about the machinery coordinating their biogenesis at the endoplasmic reticulum. Here we describe a ~ 360 kDa ribosome-associated complex comprising the core Sec61 channel and five accessory factors: TMCO1, CCDC47 and the Nicalin-TMEM147-NOMO complex. Cryo-electron microscopy reveals a large assembly at the ribosome exit tunnel organized around a central membrane cavity. Similar to protein-conducting channels that facilitate movement of transmembrane segments, cytosolic and luminal funnels in TMCO1 and TMEM147, respectively, suggest routes into the central membrane cavity. High-throughput mRNA sequencing shows selective translocon engagement with hundreds of different multi-pass membrane proteins. Consistent with a role in multi-pass membrane protein biogenesis, cells lacking different accessory components show reduced levels of one such client, the glutamate transporter EAAT1. These results identify a new human translocon and provide a molecular framework for understanding its role in multi-pass membrane protein biogenesis.

Highlights

The human genome encodes thousands of integral membrane proteins, which play critical roles in most aspects of cell physiology
We do not formally exclude a role in client-specific targeting to the ER, we propose that the TMCO1 translocon functions as an insertase and intramembrane chaperone
Our structural model for TMCO1 is consistent with its evolutionary relationship to members of the Oxa1 superfamily, including YidC, Get1, EMC3 and Ylp1 (Anghel et al, 2017)

Summary

Introduction

The human genome encodes thousands of integral membrane proteins, which play critical roles in most aspects of cell physiology. Membrane proteins of the cell surface and most intracellular compartments are first assembled at the endoplasmic reticulum Most of these are inserted by the evolutionarily conserved Sec complex, which guides hydrophobic transmembrane domains (TMDs) into a central aqueous channel that opens laterally to allow TMD entry into the bilayer (Voorhees and Hegde, 2016; Li et al, 2016; Pfeffer et al, 2015). The human genome encodes ~2500 multi-pass proteins, including GPCRs, solute carriers, ion channels, and ABC transporters These show considerable biophysical and topological complexity, including TMDs of variable length and hydrophobicity, closely spaced TMD hairpins, and re-entrant loops that span only part of the membrane (Cymer et al, 2015; Foster et al, 2000). The function of TMCO1 is not yet known, but consistent with a role in a co-translational process at the ER membrane, it can be natively isolated in association with ribosome-Sec complexes (Anghel et al, 2017)

Results

Discussion

Materials and methods