The endolysosomal system culminates in the fusion of late endosomes (LEs) with lysosomes (the vacuole in yeast), which is regulated through phosphorylation of key fusion factors by the casein kinase Yck3. Recently, an LE subpopulation has been identified in yeast with a specific role in signalling, the so-called signalling endosomes (SEs), but it is unclear what defines their identity. In this study, Lars Langemeyer, Christian Ungermann and colleagues (Grziwa et al., 2023) set out to further characterise Yck3 function and identify any novel substrates. By following the endogenously tagged kinase, they observe that Yck3 localises to both the vacuole and, surprisingly, SEs, suggesting the presence of additional substrates there. Indeed, in the absence of Yck3, the inverted (I-)BAR protein Ivy1, which has previously been found at both vacuoles and SEs, is relocalised to endosomes. Use of a non-phosphorylatable Ivy1 variant phenocopies this effect, confirming that Yck3-mediated phosphorylation controls Ivy1 localisation. The authors further map the Ivy1 sites that are important for membrane binding to a positively charged patch and the binding site for its interactor Ypt7, with mutations in either region resulting in defects in membrane biogenesis. Taken together, this work thus identifies Ivy1 as a new Yck3 substate and points to a role for Ivy1 phosphorylation in regulating endosome and vacuole homeostasis.
