Hybrid endosomal coats contain different classes of sorting nexins

TL;DR

The paper shows that different sorting nexins can form hybrid endosomal coats, allowing flexible cargo sorting in cells.

Contribution

The discovery that SNX-BARs and Snx3 can form hybrid retromer-mediated coats with variable stoichiometry is novel.

Findings

Hybrid coats assemble with variable subunit ratios and diameters.

Hybrid coats have greater membrane-scaffolding activity than homogeneous coats.

SNX3 and SNX-BARs co-localize and influence each other's cargo sorting in vivo.

Abstract

Endosomes are protein sorting stations, where multiple membrane coats form tubulovesicular carriers exporting proteins to the Golgi, the plasma membrane, or endo-lysosomal compartments. Distinct classes of sorting nexins are assumed to form distinct homogeneous coats that define the endosomal sorting routes and their cargos. Snx3 and the SNX-BAR proteins Vps5-Vps17 belong to different sorting-nexin classes. They can form homogeneous retromer-dependent coats that differ in structure and in their modes of membrane association and cargo recognition. Here, we describe the formation of hybrid coats between purified SNX-BARs, Snx3, and their cargos. Hybrid coats assemble at variable subunit ratios and diameters and show greater membrane-scaffolding activity than homogeneous coats. In vivo, Snx3 and SNX-BARs co-localise and mutually impact the sorting of their respective cargos. Although simultaneous binding of Snx3- and SNX-BARs to Retromer is sterically prohibited, hybrid coats incorporate both SNXs in a common complex, probably linked by retromer oligomerisation. We hence propose that SNX-BARs and Snx3 form retromer-mediated hybrid coats in novel, stoichiometrically adaptable configurations that allow the adjustment of endosomal carriers for transporting varying ratios of cargo.

Endosomal protein recycling is organised by distinct coat protein families that are thought to act separately, forming distinct protein carriers with distinct cargo and pathway specificity. This article shows that hybrid carriers can be formed and suggests that targeting of protein cargo may be independent of the composition of the coat forming the carrier.

Snx3 and SNX-BARs are distinct classes of sorting nexins that interact with the retromer complex to form endosomal coats with distinct architectures.Snx3 and SNX-BARs can share a common, hybrid coat that integrates cargo for both sorting nexin classes.The stoichiometric ratio of SNX-BARs over SNX3 is variable, suggesting that hybrid coats can adjust to the availability of respective cargo.Hybrid coats offer stronger membrane-scaffolding potential than the respective homogeneous SNX3 or SNX-BAR coats.The SNX3 and SNX-BAR systems are interdependent in vivo, suggesting that hybrid coats may form in cells and be necessary for correct cargo sorting.

Snx3 and SNX-BARs are distinct classes of sorting nexins that interact with the retromer complex to form endosomal coats with distinct architectures.

Snx3 and SNX-BARs can share a common, hybrid coat that integrates cargo for both sorting nexin classes.

The stoichiometric ratio of SNX-BARs over SNX3 is variable, suggesting that hybrid coats can adjust to the availability of respective cargo.

Hybrid coats offer stronger membrane-scaffolding potential than the respective homogeneous SNX3 or SNX-BAR coats.

The SNX3 and SNX-BAR systems are interdependent in vivo, suggesting that hybrid coats may form in cells and be necessary for correct cargo sorting.

Retromer recruits SNX3 and SNX-BAR adaptors to shared domains enabling flexible cargo sorting.

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