Localisation of the Schizosaccharomyces pombe rho1p GTPase and its involvement in the organisation of the actin cytoskeleton.

The Schizosaccharomyces pombe rho1p GTPase directly activates the (1-3) beta-D-glucan synthase and participates in the regulation of cell wall growth and morphogenesis in this fission yeast. Indirect immunofluorescence experiments using rho1p tagged with hemagglutinin have revealed that rho1p was located at the growing tips during interphase and at the septum prior to cytokinesis, localising to the same areas as actin patches. In S. pombe cdc10-129 mutant cells, arrested in G1, HA-rho1p accumulates at one tip whereas in cdc25-22 mutants, arrested in G2, HA-rho1p accumulates at both tips. In tea1-1 and tea2-1 cdc11-119 mutant cells, HA-rho1p is localised to the new growing tips. Overexpression of different rho1 mutant alleles caused different effects on cortical actin patch distribution, (1-3) beta-D-glucan synthase activation, and sensitivity to cell wall specific antifungal drugs. These results indicate that multiple cellular components are activated by rho1p. Overexpression of the dominant negative rho1T20N allele was lethal as was the rho1+ deletion. Moreover, when rho1+ expression was repressed in actively growing S. pombe, cells died in about 10 to 12 hours. Under these conditions, normal cell morphology was maintained but the level of (1-3) beta-D-glucan synthase activity decreased and the actin patches disappeared. Most cells lysed after cytokinesis during the process of separation, and lysis was not prevented by an osmotic stabiliser. We conclude that rho1p localisation is restricted to growth areas and regulated during the cell cycle and that rho1p is involved in cell wall growth and actin cytoskeleton organisation in S. pombe.