# SND3 is the membrane insertase within a distinct SEC61 translocon complex

## TL;DR

The study identifies SND3 as a membrane insertase in a specialized SEC61 translocon complex involved in inserting proteins into the endoplasmic reticulum membrane.

## Contribution

SND3 is identified as a membrane insertase within a distinct SEC61 translocon complex, revealing a new mechanism for co-translational insertion of multipass membrane proteins.

## Key findings

- Cryo-electron microscopy reveals a ribosome-associated SND3 translocon complex structure.
- SND3 disrupts the lipid bilayer via a hydrophilic groove to promote membrane protein insertion.
- The SND3 translocon is a distinct multipass translocon found in fungi and other eukaryotic taxa.

## Abstract

During the biogenesis of most eukaryotic integral membrane proteins (IMPs), transmembrane domains are inserted into the endoplasmic reticulum membrane by a dedicated insertase or the SEC61 translocon. The SRP-independent (SND) pathway is the least understood route into the membrane, despite catering for a broad range of IMP types. Here, we show that Chaetomium thermophilum SND3 is a membrane insertase with an atypical fold. We further present a cryo-electron microscopy structure of a ribosome-associated SND3 translocon complex involved in co-translational IMP insertion. The structure reveals that the SND3 translocon additionally comprises the complete SEC61 translocon, CCDC47 and TRAPɑ. Here, the SEC61β N-terminus works together with CCDC47 to prevent substrate access to the translocon. Instead, molecular dynamics simulations show that SND3 disrupts the lipid bilayer to promote IMP insertion via its membrane-embedded hydrophilic groove. Structural and sequence comparisons indicate that the SND3 translocon is a distinct multipass translocon in fungi, euglenozoan parasites and other eukaryotic taxa.

The fungal SND pathway inserts a wide range of proteins into the ER membrane. Here, SND3 is identified as a membrane insertase within a distinct SEC61 translocon complex, implying a role in co-translational insertion of multipass membrane proteins.