Mutual dependence of Osbp and PI4KII in the maturation of regulated secretory granules

Secretory granules (SGs) are crucial for normal animal physiology due to their role in regulated exocytosis of biologically active molecules. SG membranes are enriched in phosphatidylinositol 4-phosphate (PI4P) and cholesterol, and previous studies suggest lipid composition is important for SG biogenesis and function. Nonetheless, the molecular details of how lipids are regulated during SG biogenesis remain poorly understood. Here, we identify Oxysterol binding protein (Osbp) as a novel regulator of SG biogenesis in a Drosophila model. We show Osbp expression level positively correlates with SG size and that Osbp requires type II phosphatidylinositol 4-kinase (PI4KII) to increase SG size. Moreover, Osbp is needed for proper PI4KII and PI4P distribution, autophagic resolution and formation of cholesterol-rich endosomal tubules that are positive for PI4KII. Feeding larvae food supplemented with sterol leads to partial suppression of SG size and PI4P distribution defects in Osbp mutants. Our results indicate that reciprocal regulation of Osbp and PI4KII drives formation of membrane tubules that mediate SG maturation through elevating PI4P levels on SG membranes. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=200 SRC="FIGDIR/small/551178v1_ufig1.gif" ALT="Figure 1"> View larger version (67K): org.highwire.dtl.DTLVardef@19566fborg.highwire.dtl.DTLVardef@19beeadorg.highwire.dtl.DTLVardef@52e048org.highwire.dtl.DTLVardef@1029b3c_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIOsbp is needed for formation of PI4KII-positive endosomal tubules that enrich secretory granule membranes with PI4P and facilitate granule maturation. C_LIO_LIReciprocally, PI4KII is needed for Osbp-mediated secretory granule growth in salivary gland cells. C_LIO_LIChanges in Osbp expression levels alter autophagy initiation and autophagosome resolution in salivary gland cells. C_LIO_LIPI4KII-positive endosomal tubules are enriched in sterols, and sterol feeding suppresses defects caused by loss of Osbp. C_LI