Rab26 controls secretory granule maturation and breakdown in Drosophila

The Ykt6-Snap29-Syx13 SNARE complex promotes crinophagy via secretory granule fusion with Lamp1 carrier vesicles

In the Drosophila larval salivary gland, developmentally programmed fusions between lysosomes and secretory granules (SGs) and their subsequent acidification promote the maturation of SGs that are secreted shortly before puparium formation. Subsequently, ongoing fusions between non-secreted SGs and lysosomes give rise to degradative crinosomes, where the superfluous secretory material is degraded. Lysosomal fusions control both the quality and quantity of SGs, however, its molecular mechanism is incompletely characterized. Here we identify the R-SNARE Ykt6 as a novel regulator of crinosome formation, but not the acidification of maturing SGs. We show that Ykt6 localizes to Lamp1+ carrier vesicles, and forms a SNARE complex with Syntaxin 13 and Snap29 to mediate fusion with SGs. These Lamp1 carriers represent a distinct vesicle population that are functionally different from canonical Arl8+, Cathepsin L+ lysosomes, which also fuse with maturing SGs but are controlled by another SNARE complex composed of Syntaxin 13, Snap29 and Vamp7. Ykt6- and Vamp7-mediated vesicle fusions also determine the fate of SGs, as loss of either of these SNAREs prevents crinosomes from acquiring endosomal PI3P. Our results highlight that fusion events between SGs and different lysosome-related vesicle populations are critical for fine regulation of the maturation and crinophagic degradation of SGs.

Osh-dependent and -independent Regulation of PI4P Levels During Polarized Growth of Saccharomyces cerevisiae

Polarized secretion facilitates polarized cell growth. For a secretory vesicle to dock at the plasma membrane, it must mature with a progressive association or dissociation of molecules that are, respectively, necessary for or inhibitory to vesicle docking, including an exchange of Rab GTPases. In current models, oxysterol-binding protein homologue 4 (Osh4p) establishes a phosphatidylinositol 4-phosphate (PI4P) gradient along the secretory trafficking pathway such that vesicles have higher PI4P levels after budding from the trans-Golgi relative to when vesicles arrive at the plasma membrane. In this study, using the lipid-binding domain P4M and live-cell imaging, we show that secretory vesicle-associated PI4P levels remain constant when vesicles traffic from the trans-Golgi to the plasma membrane. We also show that deletion of OSH4 does not alter vesicle-associated PI4P levels, though loss of any individual member of the OSH family or complete loss of OSH family function alters the intracellular distribution of PI4P. We propose a model in which the Rab GTPases Ypt32p and Sec4p remain associated with a secretory vesicle during trafficking, independent of PI4P levels and Osh4p. Together these data indicate the necessity of experiments revealing the location and timing of events required for vesicle maturation.