Markers of cell polarity during and after nitrogen starvation inSchizosaccharomyces pombe

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Abstract
In Schizosaccharomyces pombe, nitrogen starvation induces transient acceleration of cell division and reduction in cell size with a final arrest in G1. The division size control appears to be impaired by mutations in cdr1/nim1 and cdr2, genes that encode protein kinases mediating nutritional control over the mitotic cycle. cdr-cells arrest after fewer rounds of division and are larger than the wild type. Recent work suggests that long-term nitrogen starvation causes S. pombe wild-type cells to become spherical, which suggests loss of cell polarity. cdr mutants retain the elongated shape, indicating a potential difference in cell polarity control relative to the wild type. We examined several markers related to maintenance of cell polarity in S. pombe following nitrogen starvation including cell division scar pattern and actin and microtubule cytoskeleton. Wild-type cells as well as cdr mutants maintained a normal cell division scar pattern throughout nitrogen starvation but cells dividing under these conditions developed a wall malformation in the center of the septum. In cells arrested by nitrogen starvation, actin patches, normally associated with sites of cell wall deposition, were larger and distributed randomly along the cell surface. Cytoplasmic arrays of microtubules, which are thought to be involved in control of the polarity signal, were not visibly affected. The effects were similar in wild-type cells and in cdr-mutants. Upon refeeding, the new growth always reoccurred at the tip zones and there were only small deviations of its direction from the original axis. The results indicate that cell polarity is preserved both in wild-type cells, which arrest in G1and appear spherical, and in cdr1/nim1 and cdr2 mutants, which arrest in G2and appear polarized throughout the starvation period. Key words: cell polarity, fission yeast, nitrogen starvation, actin, microtubules, cdr1/nim1, cdr2.