PROSPORE MEMBRANE FORMATION DEFINES A DEVELOPMENTALLY-REGULATED BRANCH OF THE SECRETORY PATHWAY IN YEAST

Spore formation in yeast is an unusual form of cell division in which the daughter cells are formed within the mother cell cytoplasm. This d ivision requires the de novo synthesis of a membrane compartment, term ed the prospore membrane, which engulfs the daughter nuclei. The effec t of mutations in late-acting genes on sporulation was investigated. M utation of SEC1, SEC4, or SEC8 blocked spore formation, and electron m icroscopic analysis of the sec4-8 mutant indicated that this inability to produce spores was caused by a failure to form the prospore membra ne. The soluble NSF attachment protein 25 (SNAP-25) homologue SEC9, by contrast, was not required for sporulation. The absence of a requirem ent for SEC9 was shown to be due to the sporulation-specific induction of a second, previously undescribed, SNAP-25 homologue, termed SPO20. These results define a developmentally regulated branch of the secret ory pathway and suggest that spore morphogenesis in yeast proceeds by the targeting and fusion of secretory vesicles to form new plasma memb ranes in the interior of the mother cell. Consistent with this model, the extracellular proteins Gas1p and Cts1p were localized to an intern al compartment in sporulating cells. Spore formation in yeast may be a useful model for understanding secretion-driven cell division events in a variety of plant and animal systems.