Sec2p is the exchange factor that activates Sec4p, the Rab GTPase controlling the final stage of the yeast exocytic pathway. Sec2p is recruited to secretory vesicles by Ypt32-GTP, a Rab controlling exit from the Golgi. Sec15p, a subunit of the octameric exocyst tethering complex and an effector of Sec4p, binds to Sec2p on secretory vesicles, displacing Ypt32p. Sec2p mutants defective in the region 450-508 amino acids bind to Sec15p more tightly. In these mutants, Sec2p accumulates in the cytosol in a complex with the exocyst and is not recruited to vesicles by Ypt32p. Thus the region 450-508 amino acids negatively regulates the association of Sec2p with the exocyst, allowing it to recycle on to new vesicles. The structures of one nearly full-length exocyst subunit and three partial subunits have been determined and, despite very low sequence identity, all form rod-like structures built of helical bundles stacked end to end. These rods may bind to each other along their sides to form the assembled complex. While Sec15p binds Sec4-GTP on the vesicle, other subunits bind Rho GTPases on the plasma membrane, thus tethering vesicles to exocytic sites. Sec4-GTP also binds Sro7p, a yeast homologue of the Drosophila lgl (lethal giant larvae) tumour suppressor. Sro7 also binds to Sec9p, a SNAP25 (25 kDa synaptosome-associated protein)-like t-SNARE [target-membrane-associated SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor)], and can form a Sec4p-Sro7p-Sec9p ternary complex. Overexpression of Sec4p, Sro7p or Sec1p (another SNARE regulator) can bypass deletions of three different exocyst subunits. Thus promoting SNARE function can compensate for tethering defects.

• PMID: 17052174
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Journal Article

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Novick P, Medkova M, Dong G, Hutagalung A, Reinisch K, Grosshans B

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