Rab26 controls secretory granule maturation and breakdown in Drosophila

Abstract

At the onset of Drosophila metamorphosis, plenty of secretory glue granules are released from salivary gland cells and the glue is deposited on the ventral side of the forming (pre)pupa to attach it to a dry surface. Prior to this, a poorly understood maturation process takes place during which secretory granules gradually grow via homotypic fusions, and their contents are reorganized. Here we show that the small GTPase Rab26 localizes to immature (smaller, non-acidic) glue granules and its presence prevents vesicle acidification. Rab26 mutation accelerates the maturation, acidification and release of these secretory vesicles as well as the lysosomal breakdown (crinophagy) of residual, non-released glue granules. Strikingly, loss of Mon1, an activator of the late endosomal and lysosomal fusion factor Rab7, results in Rab26 remaining associated even with the large glue granules and a concomitant defect in glue release, similar to the effects of Rab26 overexpression. Our data thus identify Rab26 as a key regulator of secretory vesicle maturation that promotes early steps (vesicle growth) and inhibits later steps (lysosomal transport, acidification, content reorganization, release, and breakdown), which is counteracted by Mon1.

The exocyst complex controls multiple events in the pathway of regulated exocytosis

“…Maturation of SGs is a multidimensional process that involves homotypic fusion, acidification, cargo condensation, and acquisition of membrane proteins that will steer SGs to the apical plasma membrane and contribute to the recognition, tethering and fusion (Boda et al, 2023; Ji et al, 2018; Nagy et al, 2022; Syed, Zhang, Tran, Bleck, & Ten Hagen, 2022). In the current work, we have shown that the exocyst participates in several aspects of SG maturation.…”

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

“…In addition to degradative crinosomes (at the prepupal stage -pp), ecdysone-induced progressive acidi cation also accompanies the maturation of SGs (2h before puparium formation -bpf), and is important for the remodeling of the inner structure of SGs and prepares them for exocytosis (priming) 3,[17][18][19]56,57 . Although both Vamp7 and Ykt6 proved to be required for the acidi cation of degradative crinosomes at the prepupal (pp) stage, it was still elusive whether these two R-SNAREs are also equally required for proper acidi cation of maturing SGs before their release.…”

“…The maturation and crinophagic decomposition of glue SGs both rely on a series of fusion events between SGs and lysosomes or endosomes 3,8,18,19,21,22 . Since Vamp7 and Ykt6 differentially affected SG maturation/acidi cation, we supposed that these R-SNAREs may mediate the fusion of SGs with different components of the endo-lysosomal compartment.…”

“…The nascent glue SGs emanate from the TGN 13,14 , increase in size by homotypic fusions 15,16 , and then undergo a complex maturation process during which SGs fuse with lysosomes. This promotes the acidi cation and profound reorganization of the inner content of SGs 3,[17][18][19] , preparing them for secretion 17,19,20 . Excess or abnormal glue can be also degraded by crinophagy, through fusion of non-secreted SGs and lysosomes 3,7,8,19,21,22 .…”

“…This promotes the acidi cation and profound reorganization of the inner content of SGs 3,[17][18][19] , preparing them for secretion 17,19,20 . Excess or abnormal glue can be also degraded by crinophagy, through fusion of non-secreted SGs and lysosomes 3,7,8,19,21,22 . Taken together, crosstalk and fusion between SGs and the endolysosomal compartment is critical both for SG maturation and crinosome formation, however, the molecular mechanism of these processes is still incompletely understood 3,18,19,[21][22][23] .…”