In fission yeast protein kinase C homologues (Pck1 and Pck2) are essential for cell morphogenesis. We have isolated mok1 + in a genetic screen to identify downstream effectors for Pck1/2. mok1 + is essential for viability and encodes a protein that has several membrane-spanning domains and regions homologous to glucan metabolic enzymes. mok1 mutant shows abnormal cell shape, randomization of F-actin and weak cell wall. Biochemical analysis shows that Mok1 appears to have α-glucan synthase activity. Mok1 localization undergoes dramatic alteration during the cell cycle. It localizes to the growing tips in interphase, the medial ring upon mitosis, a double ring before and dense dot during cytokinesis. Double immunofluorescence staining shows that Mok1 exists in close proximity to actin. The subcellular localization of Mok1 is dependent upon the integrity of the F-actin cytoskeleton. Conversely, overexpression of mok1 + blocks the translocation of cortical actin from one end of the cell to the other. pck2 mutant is synthetically lethal with mok1 mutant, delocalizes Mok1 and shows a lower level of α-glucan. These results indicate that Mok1 plays a crucial role in cell morphogenesis interdependently of the actin cytoskeleton and works as one of downstream effectors for Pck1/2.

A method for quantitative extraction of extravasated dye from the skin was studied in guinea pigs and rats. A simple method with a low cost and good recovery was established as follows; A piece of the skin containing extravasated dye was soaked overnight in a stoppered glass tube containing 1 ml of 1 N KOH at 37 C. Then, 9 ml of a mixed solution of 0.6 N H3PO4 and acetone (5:13) was added to the tube. The tube was shaken vigorously for a few seconds and centrifuged at 3,000 rpm for 15 min. Absorbance of supernatant was measured at 620 nm . The recovery rate of the dye was about 95% both in guinea pigs and rats .Using this method we observed that fasting stress significantly reduced the intensity of skin reactions induced by chemical mediators , heterologous PCA and especially homologous PCA in guinea pigs.

Schizosaccharomyces pombe rho1ϩ and rho2 ϩ genes are involved in the control of cell morphogenesis, cell integrity, and polarization of the actin cytoskeleton. Although both GTPases interact with each of the two S. pombe protein kinase C homologues, Pck1p and Pck2p, their functions are distinct from each other. It is known that Rho1p regulates (1,3)␤-d-glucan synthesis both directly and through Pck2p. In this paper, we have investigated Rho2p signaling and show that pck2⌬ and rho2⌬ strains display similar defects with regard to cell wall integrity, indicating that they might be in the same signaling pathway. We also show that Rho2 GTPase regulates the synthesis of ␣-d-glucan, the other main structural polymer of the S. pombe cell wall, primarily through Pck2p. Although overexpression of rho2 ϩ in wild-type or pck1⌬ cells is lethal and causes morphological alterations, actin depolarization, and an increase in ␣-d-glucan biosynthesis, all of these effects are suppressed in a pck2⌬ strain. In addition, genetic interactions suggest that Rho2p and Pck2p are important for the regulation of Mok1p, the major (1-3)␣-d-glucan synthase. Thus, a rho2⌬ mutation, like pck2⌬, is synthetically lethal with mok1-664, and the mutant partially fails to localize Mok1p to the growing areas. Moreover, overexpression of mok1 ϩ in rho2⌬ cells causes a lethal phenotype that is completely different from that of mok1 ϩ overexpression in wild-type cells, and the increase in ␣-glucan is considerably lower. Taken together, all of these results indicate the presence of a signaling pathway regulating ␣-glucan biosynthesis in which the Rho2p GTPase activates Pck2p, and this kinase in turn controls Mok1p.

Fission yeast Pat1 kinase inhibits sexual differentiation by phosphorylating the meiotic inducer Mei2 and the transcription factor Ste11. Here, we show how Pat1 downregulates these proteins. Mei2 is degraded via a ubiquitin-proteasome pathway in a phosphorylation-dependent fashion. The E2 Ubc2 and the E3 Ubr1 are required for this proteolysis. In addition, Pat1 negatively regulates Ste11 via Rad24/14-3-3, thereby repressing mei2+ transcription. The Pat1 phosphorylation sites of Ste11 match the consensus recognition sequence for 14-3-3. Rad24 binds preferentially to phosphorylated Ste11, and this binding results in inhibition of the transcriptional activation capacity of Ste11. Overall, therefore, these results show that Pat1 coordinates concerted molecular mechanisms that govern the sexual differentiation developmental decision.

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The Skp1-Cullin-1/Cdc53-F-box protein (SCF) ubiquitin ligase plays an important role in various biological processes. In this enzyme complex, a variety of F-box proteins act as receptors that recruit substrates. We have identified a fission yeast gene encoding a novel F-box protein Pof3, which contains, in addition to the F-box, a tetratricopeptide repeat motif in its N terminus and a leucine-rich-repeat motif in the C terminus, two ubiquitous protein–protein interaction domains. Pof3 forms a complex with Skp1 and Pcu1 (fission yeast cullin-1), suggesting that Pof3 functions as an adaptor for specific substrates. In the absence of Pof3, cells exhibit a number of phenotypes reminiscent of genome integrity defects. These include G2 cell cycle delay, hypersensitivity to UV, appearance of lagging chromosomes, and a high rate of chromosome loss.pof3 deletion strains are viable because the DNA damage checkpoint is continuously activated in the mutant, and this leads to G2 cell cycle delay, thereby preventing the mutant from committing lethal mitosis. Pof3 localizes to the nucleus during the cell cycle. Molecular analysis reveals that in this mutant the telomere is substantially shortened and furthermore transcriptional silencing at the telomere is alleviated. The results highlight a role of the SCFPof3 ubiquitin ligase in genome integrity via maintaining chromatin structures.

Skp1 is a central component of the E3 ubiquitin ligase SCF ( S kp1-Cullin-1-F -box). It forms an adapter bridge between Cullin-1 and the substrate-determining component, the F-box protein. In order to establish the role of Skp1, a temperature sensitive (ts) screen was carried out using mutagenic PCR (polymerase chain reaction) and 9 independent ts mutants were isolated. Mapping the mutated residues on the 3-D structure of human Skp1 suggested that the mutants would be compromised in binding to F-box proteins but not Cullin-1 (Pcu1). In order to assess the binding properties of ts Skp1, 12 F-box proteins and Pcu1 were epitopetagged, and co-immunoprecipitation performed. This systematic analysis showed that ts Skp1 retains binding to Pcu1. However, binding to three specific F-box proteins, essential Pof1, Pof3 involved in maintaining genome integrity, and nonessential Pof10, was reduced. skp1 ts cells exhibit a G2 cell cycle delay, which is attributable to activation of the DNA damage checkpoint. Intriguingly, contrary to pof3 mutants, in which this checkpoint is required for survival, checkpoint abrogation in skp1 ts suppresses a G2 delay and furthermore almost rescues the ts phenotype. The activation mechanism of the DNA damage checkpoint therefore differs between pof3∆ ∆ ∆ ∆ and skp1 ts , implicating a novel role for Skp1 in the checkpoint-signalling cascade.