Lipid Scrambling Pathways in the Sec61 Translocon Complex
| Authors | |
|---|---|
| Year of publication | 2025 |
| Type | Article in Periodical |
| Magazine / Source | Journal of the American Chemical Society |
| MU Faculty or unit | |
| Citation | |
| web | https://pubs.acs.org/doi/10.1021/jacs.4c11142 |
| Doi | http://dx.doi.org/10.1021/jacs.4c11142 |
| Keywords | Fluorescence; Free energy; Lipids; Membranes; Peptides and proteins |
| Attached files | |
| Description | Cellular homeostasis depends on the rapid, ATP-independent translocation of newly synthesized lipids across theendoplasmic reticulum (ER) membrane. Lipid translocation isfacilitated by membrane proteins known as scramblases, a few ofwhich have recently been identified in the ER. Our previousstructure of the translocon-associated protein (TRAP) bound to theSec61 translocation channel revealed local membrane thinning,suggesting that the Sec61/TRAP complex might be involved in lipidscrambling. Using complementary fluorescence spectroscopy assays,we detected nonselective scrambling by reconstituted transloconcomplexes. This activity was unaffected by Sec61 inhibitors thatblock its lateral gate, suggesting a second lipid scrambling pathwaywithin the complex. Molecular dynamics simulations indicate that the trimeric TRAP subunit forms this alternative route, facilitatinglipid translocation via a “credit card” mechanism, using a crevice lined with polar residues to shield lipid head groups from thehydrophobic membrane interior. Kinetic and thermodynamic analyses confirmed that local membrane thinning enhances scramblingefficiency and that both Sec61 and TRAP scramble phosphatidylcholine faster than phosphatidylethanolamine andphosphatidylserine, reflecting the intrinsic lipid flip-flop tendencies of these lipid species. As the Sec61 scrambling site lies inthe lateral gate region, it is likely inaccessible during protein translocation, in line with our experiments on Sec61-inhibited samples.Hence, our findings suggest that the metazoan-specific trimeric TRAP bundle is a viable candidate for lipid scrambling activity that isinsensitive to the functional state of the translocon. |
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