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bioRxiv · 10.1101/2025.07.08.663624

SND3 is the membrane insertase within a fungal multipass translocon

Yang, T.-J.·Mukherjee, S.·Langer, J. D.·Hummer, G.·McDowell, M. A.
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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 of novel 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 TRAP[a]. Here, the SEC61{beta} 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 for the insertion of multipass IMPs in fungi, euglenozoan parasites and other eukaryotic taxa.

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Yang, T.-J., Mukherjee, S., Langer, J. D., Hummer, G., McDowell, M. A.. 2025-07-11. SND3 is the membrane insertase within a fungal multipass translocon. https://doi.org/10.1101/2025.07.08.663624

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