Yck3 casein kinase-mediated phosphorylation determines Ivy1 localization and function at endosomes and vacuole

The casein kinase Yck3 is a central regulator at the vacuole that phosphorylates several proteins involved in membrane trafficking. Here, we set out to identify novel substrates. We localized endogenously tagged Yck3 not only at the vacuole, but also on endosomes. To disable Yck3 function, we generated a kinase-deficient mutant and thus identified the I-BAR-protein Ivy1 as a novel Yck3 substrate. Ivy1 localizes to both endosomes and vacuoles, and Yck3 controls this localization. A phosphomimetic Ivy1-SD mutant was found primarily on vacuoles, its non-phosphorylatable SA variant strongly localized to endosomes, similar to what is observed upon deletion of Yck3. In vitro analysis revealed that Yck3-mediated phosphorylation strongly promoted Ivy1 recruitment to liposomes carrying Rab7-like Ypt7. Modelling of Ivy1 with Ypt7 identified binding sites for Ypt7 and a positively charged patch, which are both required for Ivy1 localization. Strikingly, Ivy1 mutations in either site resulted in more cells with multilobed vacuoles, suggesting a partial defect in its membrane biogenesis. Our data thus indicate that Yck3-mediated phosphorylation controls both localization and function of Ivy1 in endolysosomal biogenesis.

The HOPS tethering complex is required to maintain signaling endosome identity and TORC1 activity

The endomembrane system of eukaryotic cells is essential for cellular homeostasis during growth and proliferation. Previous work showed that a central regulator of growth, namely the target of rapamycin complex 1 (TORC1), binds both membranes of vacuoles and signaling endosomes (SEs) that are distinct from multivesicular bodies (MVBs). Interestingly, the endosomal TORC1, which binds membranes in part via the EGO complex, critically defines vacuole integrity. Here, we demonstrate that SEs form at a branch point of the biosynthetic and endocytic pathways toward the vacuole and depend on MVB biogenesis. Importantly, function of the HOPS tethering complex is essential to maintain the identity of SEs and proper endosomal and vacuolar TORC1 activities. In HOPS mutants, the EGO complex redistributed to the Golgi, which resulted in a partial mislocalization of TORC1. Our study uncovers that SE function requires a functional HOPS complex and MVBs, suggesting a tight link between trafficking and signaling along the endolysosomal pathway.