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

The rapamycin-sensitive phosphoproteome reveals that TOR controls protein kinase A toward some but not all substrates

Regulation of cell growth requires extensive coordination of several processes including transcription, ribosome biogenesis, translation, nutrient metabolism, and autophagy. In yeast, the protein kinases Target of Rapamycin (TOR) and protein kinase A (PKA) regulate these processes and are thereby the main activators of cell growth in response to nutrients. How TOR, PKA, and their corresponding signaling pathways are coordinated to control the same cellular processes is not understood. Quantitative analysis of the rapamycin-sensitive phosphoproteome combined with targeted analysis of PKA substrates suggests that TOR complex 1 (TORC1) activates PKA but only toward a subset of substrates. Furthermore, we show that TORC1 signaling impinges on BCY1, the negative regulatory subunit of PKA. Inhibition of TORC1 with rapamycin leads to BCY1 phosphorylation on several sites including T129. Phosphorylation of BCY1 T129 results in BCY1 activation and inhibition of PKA. TORC1 inhibits BCY1 T129 phosphorylation by phosphorylating and activating the S6K homolog SCH9 that in turn inhibits the MAP kinase MPK1. MPK1 phosphorylates BCY1 T129 directly. Thus, TORC1 activates PKA toward some substrates by preventing MPK1-mediated activation of BCY1.

Protein synthesis in long-term stationary-phase cultures of Saccharomyces cerevisiae

We are interested in characterizing the process of entry into and the maintenance of the stationary phase. To identify proteins that are induced during growth to stationary phase, we examined protein synthesis in long-term stationary-phase cultures using two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). Although the total rate of protein synthesis declined when growth ceased after the postdiauxic phase, the pattern of proteins synthesized remained similar throughout the experimental period (28 days), except at the diauxic shift. At the diauxic shift most proteins detectable by 2D-PAGE undergo a transient reduction in their relative rate of synthesis that ends when cells resume growth during the postdiauxic phase. We conclude from this that the transient repression of protein synthesis at the diauxic shift is not directly associated with stationary-phase arrest. A number of proteins that are synthesized after exponential phase have been identified by 2D-PAGE. These proteins could be divided into three temporal classes depending upon when their synthesis became detectable. One postexponential protein, designated p35, was induced later than all other proteins, and its relative rate of synthesis increased throughout stationary phase. Unlike most postexponential proteins, p35 was not regulated by heat shock or glucose repression. We also observed that a direct correlation between steady-state mRNA accumulation and protein synthesis for another postexponential protein (Ssa3p) or two closely related constitutive proteins (Ssa1p and Ssa2p) did not exist. We conclude from this result that synthesis of proteins in stationary phase is regulated by mechanisms other than the control of steady-state mRNA accumulation.

Physiological analysis of mutants indicates involvement of the Saccharomyces cerevisiae GPI-anchored protein gp115 in morphogenesis and cell separation

This paper reports a phenotypic characterization of ggp1 mutants. The cloned GGP1 (GAS1) gene, which encodes a major GPI-anchored glycoprotein (gp115) of Saccharomyces cerevisiae of unknown function, was used to direct the inactivation of the chromosomal gene in haploid and diploid strains by gene replacement. The analysis of the null mutants reveals a reduction in the growth rate of 15 to 40%. Cells are round, with more than one bud, and extensively vacuolized. In the stationary phase, mutant cells are very large, arrest with a high percentage of budded cells (about 54 and 70% for haploid and diploid null mutants, respectively, in comparison with about 10 to 13% for control cells), and have reduced viability. The observed phenotype suggests defects in cell separation. Flow cytometric analysis of DNA reveals an increase in the fraction of cells in the G2+M+G1* compartment during exponential growth. Conjugation and sporulation are not affected. The exocellular location of gp115 led us to examine cell wall properties. Cell wall and septum ultrastructure of abnormally budded cells was analyzed by electron microscopy analysis, and no appreciable differences from wild-type cells were found. Microscopic analysis revealed an increase in chitin content and delocalization. In comparison with control cells, ggp1 null mutants are shown to be resistant to Zymolyase during the exponential growth phase. A fivefold overexpression of gp115 does not bring about any effects on cell growth parameters and cell wall properties.

Characterization of the cyr1-2 UGA mutation in Saccharomyces cerevisiae

cyr1-2 is a temperature-sensitive mutation of the yeast adenylate cyclase structural gene, CYR1. The cyr1-2 mutation has been suggested to be a UGA mutation since a UGA suppressor SUP201 has been isolated as a suppressor of the cyr1-2 mutation. Construction of chimeric genes restricted the region containing the cyr1-2 mutation, and the cyr1-2 UGA mutation was identified at codon 1282, which lies upstream of the region coding for the catalytic domain of adenylate cyclase. Alterations in the region upstream of the cyr1-2 mutation site result in null mutations. The complete open reading frame of the cyr1-2 gene expressed under the control of the GAL1 promoter complemented cyr1-d1 in a galactose-dependent manner. These results suggest that at the permissive temperature weak readthrough occurs at the cyr1-2 mutation site to produce low levels of active adenylate cyclase. An endogenous suppressor in yeast cells is assumed to be responsible for this readthrough.

The cell cycle modulated glycoprotein GP115 is one of the major yeast proteins containing glycosylphosphatidylinositol

The cell cycle modulated protein gp115 (115 kDa, isoelectric point about 4.8-5) of Saccharomyces cerevisiae undergoes various post-translational modifications. It is N-glycosylated during its maturation along the secretory pathway where an intermediary precursor of 100 kDa (p100), dynamically related to the mature gp115 protein, is detected at the level of endoplasmic reticulum. Moreover, we have shown by the use of metabolic labeling with [35S]methionine, [3H]palmitic acid and myo-[3H]inositol combined with high resolution two-dimensional gel electrophoresis and immunoprecipitation with a specific antiserum, that gp115 is one of the major palmitate- and inositol-containing proteins in yeast. These results, and the susceptibility of gp115 to phosphatidylinositol-specific phospholipase C treatment strongly indicate that gp115 contains the glycosylphosphatidylinositol (GPI) structure as membrane anchor domain. The two-dimensional analysis of the palmitate- and inositol-labeled proteins has also allowed the characterization of other polypeptides which possibly contain a GPI structure.

G1 events and regulation of cell proliferation

Cells prepare for S phase during the G1 phase of the cell cycle. Cell biological methods have provided knowledge of cycle kinetics and of substages of G1 that are determined by extracellular signals. Through the use of biochemical and molecular biological techniques to study effects of growth factors, oncogenes, and inhibitors, intracellular events during G1 that lead to DNA synthesis are rapidly being discovered. Many cells in vivo are in a quiescent state (G0), with unduplicated DNA. Cells can be activated to reenter the cycle during G1. Similarly, cells in culture can be shifted between G0 and G1. These switches in and out of G1 are the main determinants of post-embryonic cell proliferation rate and are defectively controlled in cancer cells.

Correlation of trehalose content and heat resistance in yeast mutants altered in the RAS/adenylate cyclase pathway: is trehalose a thermoprotectant?

Trehalose content and thermotolerance were closely correlated in wild type yeast (Saccharomyces cerevisiae) and in cyr1-2 and bcy1-1 mutants both during exponential growth at 27 degrees C and during heat shock at 40 degrees C. Trehalose levels were high when heat shock proteins (hsps) were expected to be induced and low when hsps were presumably absent. It was tried to uncouple trehalose biosynthesis and hsp-induction. Various non-heat stresses affected trehalose levels of wild type cells in a similar way as they would have affected hsps. However, no trehalose was accumulated when cells were treated with canavanine, a well-known inducer of hsps but not of the thermotolerant state.

Heat shock induction of HSP 89 is regulated in cellular aging

Induction of heat shock proteins (HSPs) was evaluated as a function of age of the IMR-90 human diploid fibroblasts. Heat shock (42 degrees C) markedly increased the synthesis of proteins with apparent molecular weights of 98, 89, 72, 50, 42 and 25 KDa, with HSP 89 and 72 being most prominent. This heat shock induction of HSPs was inversely correlated to the population doubling level (PDL) of the cell cultures used. For example, the synthesis rate of HSP 89 increased from a basal heat shock of young cells (PDL 18); in the old cells (PDL 51), the increase was from 1% to a maximum of 4% at 10-12 hrs after initiation of the heat shock. Western blot analysis showed that HSP 89 constituted approximately 2 and 10% of total cellular proteins in control and heat shocked (42 degrees C, 12-24 hrs) young cells; corresponding values for the old cells were 2 and 4.5%, respectively. Northern blot quantitation of the amount of mRNA hybridizable to cDNA probes of HSP 89 provided evidence that this age-dependent decrease in induction of HSP 89 in IMR-90 cells was attributable to a transcriptional/pre-translational mechanism.