Phosphorylation of a membrane curvature-sensing motif switches function of the HOPS subunit Vps41 in membrane tethering

Tethering factors are organelle specific multisubunit protein complexes that identify, along with Rab guanosine iriphosphatases, transport vesicles and trigger their SNARE mediated fusion of specific transport vesicles with the target membranes Little is known about how tethering factors discriminate between different trafficking pathways, which may converge at the same organelle In this paper, we describe a phosphorylation based switch mechanism, which allows the homotypic vacuole fusion protein sorting effector subunit Vps41 to operate in two distinct fusion events, namely endosome vacuole and AP 3 vesicle-vacuole fusion Vps41 contains an amphipathic lipid-packing sensor (ALPS) motif, which recognizes highly curved membranes At endosomes, this motif is inserted into the lipid bilayer and masks the binding motif for the 8 subunit of the AP 3 complex, Apl5, without affecting the Vps41 function in endosome vacuole fusion At the much less curved vacuole, the ALPS motif becomes available for phosphorylation by the resident casein kinase Yck3 As a result, the Apl5-binding site is exposed and allows AP-3 vesicles to bind to Vps41, followed by specific fusion with the vacuolar membrane This multi functional tethering factor thus discriminates between trafficking routes by switching from a curvature sensing to a coat recognition mode upon phosphorylation