Endosomal Rab GTPases regulate secretory granule maturation in Drosophila larval salivary glands

ABSTRACT

Secretory granules (SGs) are organelles responsible for regulated exocytosis of biologically active molecules in professional secretory cells. Maturation of SGs is a crucial process in which cargoes of SGs are processed and activated, allowing them to exert their function upon secretion. Nonetheless, the intracellular trafficking pathways required for SG maturation are not well defined. We recently performed an RNA interference (RNAi) screen in Drosophila larval salivary glands to identify trafficking components needed for SG maturation. From the screen, we identified several Rab GTPases (Rabs) that affect SG maturation. Expression of constitutively active (CA) and dominant-negative (DN) forms narrowed down the Rabs important for this process to Rab5, Rab9 and Rab11. However, none of these Rabs localizes to the limiting membrane of SGs. In contrast, examination of endogenously YFP-tagged Rabs (YRabs) in larval salivary glands revealed that YRab1 and YRab6 localize to the limiting membrane of immature SGs (iSGs) and SGs. These findings provide new insights into how Rab GTPases contribute to the process of SG maturation.

KEYWORDS:

• Rab GTPase
• trafficking
• secretory granule
• salivary gland
• Drosophila

Acknowledgments

We thank the Bloomington Drosophila Stock Center for fly stocks, and the Imaging Facility at The Hospital for Sick Children (P. Paroutis and K. Lau) for assistance with microscopy.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was supported by the following funding sources: SickKids Restracomp Scholarship, CIHR Strategic Training Fellowship #TGF-53877, University of Toronto Open Fellowship, and NSERC PGS-D Scholarship (to C-I. J. Ma) and CIHR Grants #PJT-162165 and #MOP-119483 (to J. A. Brill).