Liz1p, a Novel Fission Yeast Membrane Protein, Is Required for Normal Cell Division When Ribonucleotide Reductase Is Inhibited

Abstract

Ribonucleotide reductase activity is required for generating deoxyribonucleotides for DNA replication. Schizosaccharomyces pombe cells lacking ribonucleotide reductase activity arrest during S phase of the cell cycle. In a screen for hydroxyurea-sensitive mutants in S. pombe, we have identified a gene, liz1 ϩ , which when mutated reveals an additional, previously undescribed role for ribonucleotide reductase activity during mitosis. Inactivation of ribonucleotide reductase, by either hydroxyurea or a cdc22-M45 mutation, causes liz1 Ϫ cells in G2 to undergo an aberrant mitosis, resulting in chromosome missegregation and late mitotic arrest. liz1 ϩ encodes a 514-amino acid protein with strong similarity to a family of transmembrane transporters, and localizes to the plasma membrane of the cell. These results reveal an unexpected G2/M function of ribonucleotide reductase and establish that defects in a transmembrane protein can affect cell cycle progression.

Cell Division Defects of Schizosaccharomyces pombe liz1 − Mutants Are Caused by Defects in Pantothenate Uptake

“…Cdc22 is the large subunit of RNR, and the cdc22-M45 mutation causes this to be inactive at the restrictive temperature. Inactivating RNR by a temperature shift resulted in phenotypes in liz1 Ϫ cells that are the same as those observed when cells are treated with HU (28). This confirms that RNR activity is required for a normal mitosis in liz1 Ϫ cells and that this phenotype is independent of passage through S phase.…”

“…However, a proportion of liz1 Ϫ mutants undergo a highly aberrant mitosis (28). Intriguingly, when HU was added to a culture of liz1 Ϫ cells, passage through S phase was not a requirement for this defective mitosis to occur.…”

“…In synchronized G 2 cells, the first mitosis following HU addition was seen to be aberrant. In some of these cells, the new septum was seen to bisect an unseparated nucleus, a lethal event known as the "cut" phenotype (28). In less drastic cases, in the presence of HU, the liz1 Ϫ cells completed nuclear division and formed a septum but failed to decondense their chromosomes.…”

Mutations in deoxyribonucleotide biosynthesis pathway cause spreading of silencing across heterochromatic barriers at the mating‐type region of the fission yeast

“…Thus, RNR and TS together control the availability of deoxyribonucleotides for DNA synthesis in a cell. Conditional mutants of these genes, when grown under non-permissive conditions, are blocked in the G 1 -S phase (Moynihan and Enoch, 1999). Mutants described in this study also arrest at the G 1 -S phase transition when grown at 37 • C (data not shown).…”

TORC2 Is Required to Maintain Genome Stability during S Phase in Fission Yeast

“…Unexpectedly, about 40% of TORC2 mutant cells (⌬tor1) arrested in HU with septa, indicating a delay in cytokinesis. Because this delay may represent an effect of HU that is not linked with DNA damage (28,29), we set out to examine the effect of additional DNA-damaging agents on TORC2 mutant cells and to explore in more detail the DNA damage-sensitive phenotype of TORC2-Gad8 mutant cells.…”