Abstract
The exocyst complex is essential for many exocytic events, by tethering vesicles at the plasma membrane for fusion. In fission yeast, polarized exocytosis for growth relies on the combined action of the exocyst at cell poles and myosin-driven transport along actin cables. We report here the identification of fission yeast Schizosaccharomyces pombe Sec3 protein, which we identified through sequence homology of its PH-like domain. Like other exocyst subunits, sec3 is required for secretion and cell division. Cells deleted for sec3 are only conditionally lethal and can proliferate when osmotically stabilized. Sec3 is redundant with Exo70 for viability and for the localization of other exocyst subunits, suggesting these components act as exocyst tethers at the plasma membrane. Consistently, Sec3 localizes to zones of growth independently of other exocyst subunits but depends on PIP2 and functional Cdc42. FRAP analysis shows that Sec3, like all other exocyst subunits, localizes to cell poles largely independently of the actin cytoskeleton. However, we show that Sec3, Exo70 and Sec5 are transported by the myosin V Myo52 along actin cables. These data suggest that the exocyst holocomplex, including Sec3 and Exo70, is present on exocytic vesicles, which can reach cell poles by either myosin-driven transport or random walk.
Highlights
Polarized exocytosis is a fundamental biological process critical for very diverse functions, such as cell migration, polarized growth and cell-cell signaling [1,2]
Secretory vesicles are tethered by the multi-subunit exocyst complex, which is activated by Cdc42, leading to SNARE-mediated fusion of vesicles [9,10]
If SPAC17G8.12 encodes the missing Sec3 protein in fission yeast, we expected its null phenotype and sub-cellular localization to be similar to other exocyst members
Summary
Polarized exocytosis is a fundamental biological process critical for very diverse functions, such as cell migration, polarized growth and cell-cell signaling [1,2]. The budding yeast Saccharomyces cerevisiae model system has played an invaluable role in identifying many of the players and elucidating their role in polarized exocytosis [3] In this system, the master polarity regulator Cdc is activated at incipient bud sites and serves to activate members of the formin family of actin nucleators to assemble polarized arrays of actin cables [4,5,6,7]. The master polarity regulator Cdc is activated at incipient bud sites and serves to activate members of the formin family of actin nucleators to assemble polarized arrays of actin cables [4,5,6,7] These cables serve as tracks for the type V myosin mediated delivery of secretory vesicles to sites of growth [8]. Secretory vesicles are tethered by the multi-subunit exocyst complex, which is activated by Cdc, leading to SNARE-mediated fusion of vesicles [9,10]
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