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

Abstract

Eukaryotic cells depend on exocytosis to direct intracellularly synthesized material towards the extracellular space or the plasma membrane, so exocytosis constitutes a basic function for cellular homeostasis and communication between cells. The secretory pathway includes biogenesis of secretory granules (SGs), their maturation and fusion with the plasma membrane (exocytosis), resulting in release of SG content to the extracellular space. The larval salivary gland of Drosophila melanogaster is an excellent model for studying exocytosis. This gland synthesizes mucins that are packaged in SGs that sprout from the trans-Golgi network and then undergo a maturation process that involves homotypic fusion, condensation and acidification. Finally, mature SGs are directed to the apical domain of the plasma membrane with which they fuse, releasing their content into the gland lumen. The exocyst is a hetero-octameric complex that participates in tethering of vesicles to the plasma membrane during constitutive exocytosis. By precise temperature-dependent gradual activation of the Gal4-UAS expression system, we have induced different levels of silencing of exocyst complex subunits, and identified three temporarily distinctive steps of the regulated exocytic pathway where the exocyst is critically required: SG biogenesis, SG maturation and SG exocytosis. Our results shed light on previously unidentified functions of the exocyst along the exocytic pathway. We propose that the exocyst acts as a general tethering factor in various steps of this cellular process. O_FIG O_LINKSMALLFIG WIDTH=124 HEIGHT=200 SRC="FIGDIR/small/555734v2_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@4057f5org.highwire.dtl.DTLVardef@4ed694org.highwire.dtl.DTLVardef@1e9a859org.highwire.dtl.DTLVardef@12b971f_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical abstract.C_FLOATNO Proposed model of the action of the exocyst in maintenance of normal Golgi complex structure, maturation and exocytosis of secretory granules in Drosophila larval salivary gland cells. 1-Before secretory granule (SG) biogenesis (<96 hours AEL), the exocyst (pink dots) localizes at the Golgi complex, where it is required to maintain the normal Golgi structure. The mucine Sgs3 (brown dots) moves through the secretory pathway from the endoplasmic reticulum (ER) to the Golgi complex, from where immature SGs containing the mucine sprout out. 2-After sprouting, SGs undergo maturation (96-116 hours AEL). During maturation, the exocyst localizes in between immature SGs, where it is required for homotypic fusion. The exocyst is also required for incorporation of maturation factors to the membrane of SGs. These maturation factors include Syt-1 (purple line), DC63 (orange line), Rab11 (green oval) and Rab1 (light blue oval). At this stage, the exocyst no longer localizes at the Golgi complex. 3-When maturation has been completed SGs fuse with the apical plasma membrane and exocytosis takes place. During exocytosis (116-120 hours AEL), the exocyst localizes at mature SGs, in contact with the apical plasma membrane, where it is required for tethering and subsequent fusion, prior to release of the SG content to the salivary gland lumen. C_FIG