Modulation of expression of the stress-inducible p118 of Saccharomyces cerevisiae by cAMP. II. A study of p118 expression in mutants of the cAMP cascade

In the preceding paper, we have identified a protein of Mr = 118,000 which is induced by stress conditions that lead to cessation of DNA synthesis and cell division (Verma, R., Iida, H., and Pardee, A.B. (1988) J. Biol. Chem. 263, 8569-8575). In the current study, we have investigated the possible role this protein may play in cellular proliferation by studying p118 expression in mutants of the cAMP metabolic pathway. The cyr 1-2 mutant gene encodes a thermolabile adenylate cyclase whose activity is only 7% of wild type even at permissive temperatures (23 degrees C). We have found that at 23 degrees C, the G1 period was 5-fold longer in cyr 1-2 than in CYR1+ cells and that p118 was constitutively expressed in these slow cycling mutants. Addition of 8-bromo-cAMP to cyr 1-2 mutants restored growth at both the restrictive and permissive temperatures and resulted in a shut-off in the synthesis of p118. The effect of the analog on p118 expression was rapid, preceding the increase in cell number and percentage-budded cells. In contrast to wild type cells, p118 synthesis was not induced by sulfur starvation in RAS2val19 mutants possessing high levels of adenylate cyclase activity and bcy1 mutants defective in the regulatory subunit of cAMP-dependent protein kinase. A large body of evidence exists supporting a role of cAMP in positive control of cell proliferation. It is therefore possible that conditions which decrease cAMP arrest growth through a chain of events that include p118 induction.