A novel Cdc20-related WD-repeat protein, Fzr1, is required for spore formation in Schizosaccharomyces pombe

Ste9/Srw1 which shows sequence homology to Hctl from budding yeast, is an activator of the anaphase-promoting complex (APC) in the fission yeast Schizosaccharomyces pombe. By homology search of the S. pombe genome, we identified the gene fr1+, which encodes the protein with the highest homology to Ste9 among five Cdc20-like proteins. Like Ste9, Fzr1 contains seven WD-repeats in its C-terminal region. In spite of this structural similarity, however, overproduction of either of these proteins cannot complement mutants lacking the other. fzr1+ is transcribed exclusively during meiosis and sporulation, suggesting that it plays a role in these processes. In fact, the fzr1 disruptant formed aberrant asci, which contained only one or two mature spores, though meiotic nuclear divisions proceeded with kinetics similar to wild type, and meiotic segregation of chromosomes was normal. Structural alteration of spindle pole bodies, which is a prerequisite for the formation of the forespore membrane, occurred normally in fzr1delta during the second meiotic division. Localization of spore rim marker proteins fused to green fluorescent protein showed that nascent prespores were irregularly shaped, small in size and few in number in fzr1delta cells compared to wild-type cells. Furthermore, electron microscopy revealed that the outer layer of the spore walls was often missing in fzr1delta spores. These results show that Fzr1 is specifically involved in the assembly of the spore envelope and also in spore maturation. Fzr1, a structural homolog of the APC regulator, therefore plays an important role in spore morphogenesis.