N-terminal signals in the SNX-BAR paralogs Vps5 and Vin1 guide coat complex formation

Abstract

Endosomal coat complexes assemble by incorporating membrane-binding subunits such as those of the sorting nexin (SNX) family. The S. cerevisiae SNX-BAR paralogs Vin1 and Vps5 are respective subunits of the endosomal VINE and retromer complexes that arose from a fungal whole genome duplication. Interactions mediated by the Vin1 and Vps5 BAR domains are required for protein complex assembly and membrane association. However, a degree of promiscuity is predicted for yeast BAR-BAR pairings, suggesting that another mechanism guides the formation of specific endosomal coat complexes. Previous work by our group and others has implicated the unstructured N-terminal domains of Vin1 and Vps5 in complex assembly. Here, we map N-terminal signals in both SNX-BAR paralogs that contribute to the formation and function of two distinct endosomal coats in vivo. Whereas Vin1 leverages a polybasic region and adjacent hydrophobic motif to bind Vrl1 and form VINE, the N-terminus of Vps5 interacts with the retromer subunit Vps29 at two separate sites. We show that one of these Vps5 motifs binds to a conserved hydrophobic pocket in Vps29 that is shared with other accessory proteins and targeted by a bacterial virulence factor in humans. Lastly, we examined the sole isoform of Vps5 from the milk yeast K. lactis and found that ancestral yeasts may have used a nested N-terminal signal to form both VINE and retromer. Our results suggest that the specific assembly of Vps5-family SNX-BAR coats depends on inputs from unique N-terminal sequence features in addition to BAR domain coupling, expanding our understanding of endosomal coat assembly mechanisms.