An ER translocon for multi-pass membrane protein biogenesis

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. Cryoelectron 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.

Time-Resolved Interactome Profiling Deconvolutes Secretory Protein Quality Control Dynamics

“…Conversely, WT interactions peaked at the 0 hr time point and consistently tapered off for many of these components, with DNAJC3 and SDF2L1 mainly being enriched at the 0 hr time point (Figure 3D; Figures S3 and S4). We even noticed altered temporal interactions for previously unidentified components including EMC1 and EMC4 of the ER membrane complex (EMC) and CCDC47 of the PAT complex, all of which are involved membrane protein processing within the ER (Figures 3E; Figure S4) (Chitwood and Hegde, 2020; McGilvray et al, 2020a; Shurtleff et al, 2018). WT Tg interactions with both CCDC47 and EMC1 peak at the end of the chase period, yet for C1264R this temporal profile is inversed peaking at the 0 hr time point and slowly tapering off.…”

Defining the Ligand-dependent Interactome of the Sigma 1 Receptor

“…3A)(32,33). Multi-pass transmembrane proteins are imported into the ER with the aid of the Nicalin-TMEM147-NOMO complex(34). Numerous proteins within the S1R interactome function at various steps in the ER translocation process (Fig.…”

Bi-allelic loss-of-function variants in TMEM147 cause moderate to profound intellectual disability with facial dysmorphism and pseudo-Pelger-Huët anomaly

“…Ile133 is located at the beginning of a-helix 5 between the transmembrane domains 3 and 4, while Arg166 is located at the beginning of a-helix 6 where the transmembrane region initiates (Figure S1). The 3D structure of the translocon was recently resolved by cryo-electron microscopy (PDB: 6W6L, Figure 3A) 9 and used here to further explore the possible functional impact of the three missense changes (Figures 3B and 3C). Considering the p.Gly7Arg substitution, the arginine extension was predicted to push into the TMCO1-facing amino terminal region, creating a steric conflict with the Phe96 (Figure 3, insert 2, asterisks).…”

“…7,8 A recent study identified a 360 kDa ribosome-associated complex comprising the core Sec61 (translocon) channel and the five accessory factors, transmembrane and coiledcoil domains 1 (TMCO1), coiled-coil domain-containing protein 47 (CCDC47), and the nicalin-TMEM147-NOMO complex, localized at the ribosome exit tunnel, organized around a central membrane cavity revealed by cryo-electron microscopy. 9 The translocon is a highly conserved multi-subunit protein complex that consists of three subunits (Sec61a, Sec61b, and Sec61g) functioning as a protein-conducting channel connecting the cytoplasmic and luminal spaces on either side of the ER membrane. Within the translocon, pathogenic variants involved in a human disease have so far only been identified in SEC61A1 (MIM: 609213), which encodes one of the Sec61a subunits of the translocon complex, generating an autosomal-dominantinherited condition named tubulointerstitial kidney disease 5 (MIM: 617056), responsible for nephropathy with ID.…”