Figure 13

Model of the function of the exocyst complex during defense. (a) The exocyst is composed of 8 subunits: EXOC1, EXOC2, EXOC3, EXOC4, EXOC5 and EXOC6, EXOC7 and EXOC8. In S. cerevisiae, Supressor of Sec One (Sso1p) and Sec9p (SNAP-25) are target membrane SNARE proteins that bind synaptobrevin homolog 1 (Snc1p) to bring the target and vesicle membranes closer. Sec4p is a vesicle membrane, Ras-related, GTPase that binds the exocyst. EXOC7 and EXO1C bind the target membrane by the positively charged residues of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) (shown as blue triangles with a white +). Adapted from⁴⁴. The process results in tethering, docking and membrane fusion, releasing cargo contents. (b) In the absence of even one exocyst component, the rest of the components do not function properly, impairing vesicle tethering. This impairment inhibits vesicle docking and, thus, target and membrane fusion events do hot happen, so the vesicle contents remain in the untethered vesicle.