Essential function of Drosophila Sec6 in apical exocytosis of epithelial photoreceptor cells

Abstract

Polarized exocytosis plays a major role in development and cell differentiation but the mechanisms that target exocytosis to specific membrane domains in animal cells are still poorly understood. We characterized Drosophila Sec6, a component of the exocyst complex that is believed to tether secretory vesicles to specific plasma membrane sites. sec6 mutations cause cell lethality and disrupt plasma membrane growth. In developing photoreceptor cells (PRCs), Sec6 but not Sec5 or Sec8 shows accumulation at adherens junctions. In late PRCs, Sec6, Sec5, and Sec8 colocalize at the rhabdomere, the light sensing subdomain of the apical membrane. PRCs with reduced Sec6 function accumulate secretory vesicles and fail to transport proteins to the rhabdomere, but show normal localization of proteins to the apical stalk membrane and the basolateral membrane. Furthermore, we show that Rab11 forms a complex with Sec5 and that Sec5 interacts with Sec6 suggesting that the exocyst is a Rab11 effector that facilitates protein transport to the apical rhabdomere in Drosophila PRCs.

Early to Intermediate Steps of Tumor Embolic Formation Involve Specific Proteolytic Processing of E-Cadherin Regulated by Rab7

“…Exoc5 was upregulated and Hgs and Rab7 were downregulated in the MARY-X spheroids. Exoc5 (Sec10 in Drosophila) is thought to jointly regulate trafficking of E-cadherin to the epithelial cell surface through interaction with ExoC6 (Sec15 in Drosophila), Exoc2 (Sec5 in Drosophila), GTP-bound Rab11, and b-catenin (12)(13)(14)(15) and therefore its increase in MARY-X spheroids contributes to E-cadherin trafficking to the membrane. Hgs, one of the master regulators that direct activated receptors toward lysosomes, has been shown to play an important role in the lysosomal targeting of E-cadherin (16).…”

Exocyst Requirement for Endocytic Traffic Directed Toward the Apical and Basolateral Poles of Polarized MDCK Cells

“…Although Sec6 and Sec8 showed little association with EEA1, Exo70 appeared to distribute to the periphery of these early endosomal structures. Work in Drosophila has previously established that the distribution of individual exocyst components is not always identical (Beronja et al, 2005;Murthy et al, 2005). The function of Exo70 in early endosomes is unknown but the absence of Sec6 or Sec8 may indicate that Exo70 is acting independently or as part of a subcomplex.…”

“…Although the exocyst is generally thought to be involved in promoting transit between intracellular compartments and the plasma membrane, some studies indicate that it may also play a role in modulating cargo exit from the TGN or endosomes (Yeaman et al, 2001;Beronja et al, 2005;Langevin et al, 2005). To explore this possibility, we reconstituted vesicle budding from ARE in mechanically perforated cells.…”

“…Structural analysis has defined the site of Drosophila Rab11 interaction to a single helix in the C-terminal region of Sec15 (Wu et al, 2005). The functional significance of the Sec15/Rab11 interaction is not well characterized, but overexpression of Sec15A-GFP in COS cells slows the egress of Tf from recycling endosomes , whereas in Drosophila, mutations in Sec5, Sec6, or Sec15 result in the accumulation of cargo in enlarged Rab11-positive endosomes (Beronja et al, 2005;Langevin et al, 2005). Rab11 is localized to the TGN and recycling endosomes of nonpolarized cells (Ullrich et al, 1996;Ren et al, 1998;Wilcke et al, 2000).…”

“…The exocyst is localized to the primary cilium (Rogers et al, 2004); it regulates Ca 2ϩ signaling at the apical domain of pancreatic acinar cells (Shin et al, 2000); it is required for targeting secretory vesicles to the rhabdomere (a densely packed tuft of microvilli located at the apical pole of the insect photoreceptor cells; Beronja et al, 2005); it regulates exocytosis of apical secretory proteins in MDCK cells (Lipschutz et al, 2000); it is associated with aquaporin-2 containing vesicles (which recycle at the apical pole of collecting duct principal cells; Barile et al, 2005), and it may be involved in apical trafficking of Spdo/Notch/Delta (JafarNejad et al, 2005). Intriguingly, the exocyst may also modulate a form of "transcytosis," whereby DE-cadherin is delivered from the lateral membranes to the adherens junctions localized above the septate junction of insect epithelial cells (Langevin et al, 2005).…”