DNA-REPLICATION AND ORDER OF CELL-CYCLE EVENTS - A ROLE FOR PROTEIN ISOPRENYLATION

When the aya1(+) gene is mutated, Schizosaccharomyces pombe cells beco me unable to react appropriately to a delay in DNA replication. Instea d of stalling the cell cycle to allow completion of DNA synthesis, the y proceed unperturbed towards mitosis and attempt to segregate the sti ll unreplicated chromosomes. As a result, the genetic material segrega tes unevenly and the nuclei assume a mitotic catastrophe phenotype, ch aracterized by torn chromosomes (cut), anucleated cells and scattered chromosomes. Interestingly, the aya1 phenotype can be suppressed by ov erexpression of either the catalytic subunit of S. pombe DNA polymeras e alpha or of a novel protein called hur1(+)p. The tatter bears signif icant homology to the core of the human Rab escort protein, which belo ngs to a family of factors necessary to the post-translational isopren ylation of proteins like Ras, nab and lamin B. When isoprenylation is chemically inhibited with R-limonene (a monoterpene derived from orang e rind), wild type S. pombe cells become insensitive to an S phase del ay, in a manner strongly reminiscent of aya1 mutants. Moreover, overex pression of hur1(+)p in wild type cells rescues the failing checkpoint function. We propose that there is a strong correlation between the a ya1 phenotype, S-M phase checkpoint function, and isoprenytation event s in fission yeast.