PIKfyve influences inter-organelle contacts with lysosomes to modulate the endoplasmic reticulum

Lysosomes clear unwanted cellular material delivered by constant membrane fusion. Membrane fission is thus required to balance lysosome size, number, and composition. PIKfyve is a lipid kinase that converts phosphatidylinositol-3-phosphate [PtdIns(3)P] to phosphatidylinositol-3,5-bisphosphate [PtdIns(3,5)P2] and promotes lysosome fission since lysosomes coalesce into larger, but fewer organelles in its absence. Here, we reveal a role for PIKfyve in regulating endoplasmic reticulum (ER) dynamics. We show the ER is less reticulated and motile in cells inhibited for PIKfyve. Partly, this arises because lysosomes cluster perinuclearly and are less motile, which appears to arrest ER hitchhiking, a process in which lysosomes pull and form ER tubules. Secondly, the ER morphology is distorted because of hyper-tethering of protrudin, an ER transmembrane protein, to lysosomes via excess PtdIns(3)P and protrudins FYVE domain. Our findings reveal that PIKfyve balances phosphoinositides at ER-lysosome contact sites to govern ER properties and have significant implications for our understanding of PIKfyve function and of diseases linked to its dysfunction. SummaryLong associated with the regulation of the endolysosomal pathway, the authors show that the PIKfyve lipid kinase extends its sphere of influence to modulate the architecture and dynamics of the endoplasmic reticulum by balancing phosphoinositide species at their inter-organellar sites.