The architecture of EMC reveals a path for membrane protein insertion
- MRC Laboratory of Molecular Biology, United Kingdom
- Diamond Light Source, United Kingdom
- University of Chicago, United States
Abstract
Approximately 25% of eukaryotic genes code for integral membrane proteins that are assembled at the endoplasmic reticulum. An abundant and widely conserved multi-protein complex termed EMC has been implicated in membrane protein biogenesis, but its mechanism of action is poorly understood. Here, we define the composition and architecture of human EMC using biochemical assays, crystallography of individual subunits, site-specific photocrosslinking, and cryo-EM reconstruction. Our results suggest that EMC's cytosolic domain contains a large, moderately hydrophobic vestibule that can bind a substrate's transmembrane domain (TMD). The cytosolic vestibule leads into a lumenally-sealed, lipid-exposed intramembrane groove large enough to accommodate a single substrate TMD. A gap between the cytosolic vestibule and intramembrane groove provides a potential path for substrate egress from EMC. These findings suggest how EMC facilitates energy-independent membrane insertion of TMDs, explain why only short lumenal domains are translocated by EMC, and constrain models of EMC's proposed chaperone function.
Data availability
Structural coordinates have been deposited in PDB under accession codes 6Y4L and 6Z3W. Cryo-EM data had been deposited to EMDB under accession code EMD-11058. All other data in this study are provided within the manuscript.
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Crystal structure of the human EMC2•EMC9 complexProtein Data Bank, 6Y4L.
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Structure of the idle mammalian ribosome-Sec61 complexProtein Data Bank, 3J7Q.
Article and author information
Author details
Szymon Juszkiewicz
Elizabeth A Miller
Funding
Medical Research Council (MC_UP_A022_1007)
- Ramanujan S Hegde
Medical Research Council (MC_UP_1201/10)
- Elizabeth A Miller
National Institutes of Health (R01 GM078186)
- Elizabeth A Miller
National Institutes of Health (R01 GM130051)
- Robert J Keenan
European Molecular Biology Organization (ALTF 18-2018)
- John P O'Donnell
Boehringer Ingelheim Fonds
- Ben Photon Phillips
Naito Foundation
- Yuichi Yagita
Japanese Biochemical Society
- Yuichi Yagita
Swiss National Science Foundation (P2ELP3_18910)
- Duccio Malinverni
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
Copyright
© 2020, O'Donnell et al.
This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.
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The architecture of EMC reveals a path for membrane protein insertion
eLife 9:e57887.
https://doi.org/10.7554/eLife.57887
Further reading
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- Biochemistry and Chemical Biology
- Microbiology and Infectious Disease
Teichoic acids (TA) are linear phospho-saccharidic polymers and important constituents of the cell envelope of Gram-positive bacteria, either bound to the peptidoglycan as wall teichoic acids (WTA) or to the membrane as lipoteichoic acids (LTA). The composition of TA varies greatly but the presence of both WTA and LTA is highly conserved, hinting at an underlying fundamental function that is distinct from their specific roles in diverse organisms. We report the observation of a periplasmic space in Streptococcus pneumoniae by cryo-electron microscopy of vitreous sections. The thickness and appearance of this region change upon deletion of genes involved in the attachment of TA, supporting their role in the maintenance of a periplasmic space in Gram-positive bacteria as a possible universal function. Consequences of these mutations were further examined by super-resolved microscopy, following metabolic labeling and fluorophore coupling by click chemistry. This novel labeling method also enabled in-gel analysis of cell fractions. With this approach, we were able to titrate the actual amount of TA per cell and to determine the ratio of WTA to LTA. In addition, we followed the change of TA length during growth phases, and discovered that a mutant devoid of LTA accumulates the membrane-bound polymerized TA precursor.
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