Adenylyl cyclase activity of the fission yeast Schizosaccharomyces pombe is not regulated by guanyl nucleotides

In the budding yeast Kluyveromyces marxianus, adenylate cyclase is regulated by Ras protein(s) in vitro

The presence of adenylate cyclase activity was first demonstrated in membrane fractions from the budding yeast Kluyveromyces marxianus. The enzyme showed a Mn(2+)- and Mg(2+)-dependent activity, with optimal pH at around 6 as observed in other yeast species. As in Saccharomyces cerevisiae, where adenylate cyclase is regulated by RAS1 and RAS2, we detected a guanyl nucleotide-dependent activity. Interestingly Y13-259 monoclonal antibody, raised against mammalian p21Ha-ras, inhibited Mg2+ plus GTP-gamma-S-dependent cAMP production, suggesting that the GTP binding proteins involved in adenylate cyclase regulation could be Ras proteins. The same antibody recognized on Western blot and immunoprecipitated a 40 kDa polypeptide from K. marxianus crude membranes. This polypeptide was not detected by an anti-RAS2 polyclonal antibody raised against S. cerevisiae RAS2 protein, suggesting that Ras proteins from the two species could be structurally different.

Glucose repression of transcription of the Schizosaccharomyces pombe fbp1 gene occurs by a cAMP signaling pathway

Transcription of the fbp1 gene, encoding fructose-1,6-bisphosphatase, of Schizosaccharomyces pombe is subject to glucose repression. Previous work has demonstrated that several genes (git genes) are required for this repression. In this report we demonstrate that one of these genes, git2, is the same as the cyr1 gene, which encodes adenylate cyclase, and that loss-of-function mutations in git2 cause constitutive fbp1 transcription. Addition of cAMP to the growth medium suppresses the transcriptional defect in git2 mutants as well as in strains that carry mutations in any of six additional git genes. Similarly, exogenous cAMP represses fbp1 transcription in wild-type cells grown on a derepressing carbon source. Different levels of adenylate cyclase activity in different git2 mutants, coupled with the result that some git2 mutants display intragenic complementation, strongly suggest that adenylate cyclase acts as a multimer and that different git2 mutations alter distinct activities of adenylate cyclase, including catalytic activity and response to glucose. Additional experiments demonstrate that this cAMP signaling pathway is independent of the S. pombe ras1 gene and works by activation of cAMP-dependent protein kinase.

Adenylyl cyclase is dispensable for vegetative cell growth in the fission yeast Schizosaccharomyces pombe

Disruption of the cyr1 gene of Schizosaccharomyces pombe, which encodes adenylyl cyclase, did not confer lethality to fission yeast cells, although they grew 40% slower than wild-type strains in complete medium. These cells contained no measurable amount of cAMP and no adenylyl cyclase activity. When h+ and h- cyr1 disruptants were mixed, they underwent mating even in rich medium. Propagation of homothallic cyr1 disruptants was difficult, probably because such cells readily mate and produce asci and thus stop growing. A greater than 10-fold increase in the amount of cyr1 mRNA was observed when cloned cyr1+ was introduced into Sch. pombe cells on a multicopy plasmid. The total adenylyl cyclase activity was similarly high in these transformants. However, the level of intracellular cAMP was hardly affected. Evidence suggests that this was not due to increased phosphodiesterase activity. Thus, cAMP level in growing fission yeast cells appears to be regulated not by the amount of adenylyl cyclase protein but by a feedback mechanism at the enzyme level. The cAMP level fell by approximately 50% under nitrogen starvation, which triggers sexual development in Sch. pombe. We suggest that fission yeast controls the level of intracellular cAMP primarily to regulate sexual development rather than to drive or arrest the cell cycle.