A MUTATION IN THE RCC1-RELATED PROTEIN PIM1 RESULTS IN NUCLEAR-ENVELOPE FRAGMENTATION IN FISSION YEAST

Members of the RCC1 protein family are chromatin-associated guanine nu cleotide exchange factors that have been implicated in diverse cellula r processes in various organisms, yet no consensus has been reached as to their primary biological role. The fission yeast Schizosaccharomyc es pombe, a single-celled eukaryote, provides an in vivo system in whi ch to study the RCC1/Ran switch by using a temperature-sensitive mutan t in the RCC1-related protein pim1, Mitotic entry in the pim1-d1(ts) m utant is normal, but mitotic exit leads to the accumulation of cells a rrested with a medial septum and condensed chromosomes, Although the y east nuclear envelope normally remains intact throughout the cell cycl e, we found a striking fragmentation of the nuclear envelope in the pi m1-d1(ts) mutant following mitosis, This resulted in chromatin that wa s no longer compartmentalized and an accumulation of pore-containing m embranes in the cytoplasm, The development of this terminal phenotype was dependent on the passage of cells through mitosis and was coincide nt with the loss of viability. We propose that pim1 is required for th e reestablishment of nuclear structure following mitosis in fission ye ast.