Cabrera et al. reveal how phosphorylation alters the membrane-tethering activity of Vps41, allowing the protein to operate in two distinct trafficking pathways.
Vps41 is part of the HOPS complex that controls transport to the yeast vacuole from two different sources, tethering either endosomes or Golgi-derived AP-3–coated vesicles to the vacuolar membrane. The vacuolar kinase Yck3 may help the HOPS complex discriminate between these pathways by phosphorylating Vps41. In the absence of Yck3, Vps41 accumulates on endosomes and targets them to the vacuole successfully, but it doesn't function in the delivery of AP-3 vesicles to the vacuole.
Cabrera et al. discovered that Yck3 phosphorylates Vps41 in a membrane-binding motif called an amphipathic lipid-packing sensor (ALPS). The ALPS is an α-helical structure that inserts into highly curved membranes. Insertion of the Vps41 ALPS into endosomal membranes masks a binding site for the AP-3 coat, the team found, preventing the fusion of AP-3 vesicles with endosomes but allowing Vps41 to deliver endosomes to the vacuole. Once at the vacuole, however, Yck3 phosphorylates Vps41 in its ALPS motif. Cabrera et al. found that this helped release the motif from membranes, making Vps41’s AP-3–binding site available to tether AP-3 vesicles to the vacuole.
Yck3 therefore switches Vps41 from its function in endosome–vacuole fusion to its role in AP-3 vesicle delivery. Author Christian Ungermann now wants to investigate if similar principles apply to other tethering processes and to determine what happens after Vps41 recognizes the incoming AP-3 vesicle—the vesicle must quickly shed its coat to allow vacuole fusion to proceed.