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

Ste9/Srw1, which shows sequence homology to Hct1 from budding yeast, is an activator of the anaphase-promoting complex (APC) in the fission yeast Schi zosaccharomyces,ces pombe. By homology search of the S. pombe genome, we id entified the gene fzr1(+), which encodes the protein with the highest homol ogy 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 mut ants lacking the other. fzr1(+) is transcribed exclusively during meiosis a nd 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 si milar to wild type, and meiotic segregation of chromosomes was normal. Stru ctural alteration of spindle pole bodies, which is a prerequisite for the f ormation of the forespore membrane, occurred normally in fzr1 Delta during the second meiotic division. Localization of spore rim marker proteins fuse d to green fluorescent protein shoved that nascent prespores were irregular ly shaped, small in size and few in number in fzr1 Delta cells compared to wild-type cells. Furthermore, electron microscopy revealed that the outer l ayer of the spore walls was often missing in fzr1 Delta spores. These resul ts show that Fzr1 is specifically involved in the assembly of the spore env elope and also in spore maturation. Fzr1, a structural homolog of the APC r egulator, therefore plays an important role in spore morphogenesis.