The Anaphase Promoting Complex Targeting Subunit Ama1 Links Meiotic Exit to Cytokinesis during Sporulation inSaccharomyces cerevisiae

Abstract

Ascospore formation in yeast is accomplished through a cell division in which daughter nuclei are engulfed by newly formed plasma membranes, termed prospore membranes. Closure of the prospore membrane must be coordinated with the end of meiosis II to ensure proper cell division. AMA1 encodes a meiosis-specific activator of the anaphase promoting complex (APC). The activity of APC(Ama1) is inhibited before meiosis II, but the substrates specifically targeted for degradation by Ama1 at the end of meiosis are unknown. We show here that ama1Delta mutants are defective in prospore membrane closure. Ssp1, a protein found at the leading edge of the prospore membrane, is stabilized in ama1Delta mutants. Inactivation of a conditional form of Ssp1 can partially rescue the sporulation defect of the ama1Delta mutant, indicating that an essential function of Ama1 is to lead to the removal of Ssp1. Depletion of Cdc15 causes a defect in meiotic exit. We find that prospore membrane closure is also defective in Cdc15 and that this defect can be overcome by expression of a form of Ama1 in which multiple consensus cyclin-dependent kinase phosphorylation sites have been mutated. These results demonstrate that APC(Ama1) functions to coordinate the exit from meiosis II with cytokinesis.

Sporulation in the Budding Yeast Saccharomyces cerevisiae

“…As the membrane grows, it does so with a characteristic curvature and in the appropriate direction so that it can engulf the forming daughter nucleus (Figure 3,D and E). Videomicroscopy studies demonstrate that the membranes initially appear as small horseshoes that become small circles before abruptly expanding into long cylindrical tubes (Diamond et al 2008). This transition may correspond to the lengthening of the meiosis II spindle during anaphase.…”

“…This transition may correspond to the lengthening of the meiosis II spindle during anaphase. These tubes then round into ovals before returning to a spherical shape coincident with membrane closure (Diamond et al 2008). Both membrane-associated cytoskeletal elements and components of the membrane itself are required to control this stereotyped growth pattern of the membrane.…”

“…The leading edge complex is present throughout prospore membrane growth, and then it breaks down just prior to membrane closure due to proteolysis of Ssp1 (Maier et al 2007;Diamond et al 2008). In addition to its positive role in proper membrane growth, Ssp1 plays a negative role in regulating the closure of the prospore membrane (Maier et al 2007).…”

“…Premature closure could cause either complete failure to capture nuclei or nuclear fragmentation. Membrane closure is coordinated with meiotic exit through the action of the anaphase promoting complex (APC) and its regulatory subunit Ama1 (Diamond et al 2008). The APC is an E3 ubiquitin ligase that is directed to specific substrates through regulators of the Cdc20 family (Vodermaier 2001).…”

“…Though ama1D cells complete the meiotic divisions (as judged by the appearance of four distinct DAPI staining nuclei) and form prospore membranes, no spores are formed and markers for spore wall formation are absent (Coluccio et al 2004a). A fluorescence loss in photobleaching assay revealed that in postmeiotic ama1 mutants the presumptive spore and ascal cytoplasms remain connected (Diamond et al 2008); i.e., the prospore membranes fail to close. This phenotype is suppressed by a temperature-sensitive ssp1 allele, implying that the defect reflects stabilization of Ssp1 (Diamond et al 2008).…”