Connections between the Ras-cyclic AMP pathway and G1 cyclin expression in the budding yeast Saccharomyces cerevisiae.

Abstract

We have identified two processes in the G1 phase of the Saccharomyces cerevisiae cell cycle that are required before nutritionally arrested cells are able to return to proliferative growth. The first process requires protein synthesis and is associated with increased expression of the G1 cycin gene CLN3. This process requires nutrients but is independent of Ras and cyclic AMP (cAMP). The second process requires cAMP. This second process is rapid, is independent of protein synthesis, and produces a rapid induction of START-specific transcripts, including CLNI and CLN2. The ability of a nutritionally arrested cell to respond to cAMP is dependent on completion of the first process, and this is delayed in cells carrying a CLN3 deletion. Mating pheromone blocks the cAMP response but does not alter the process upstream of Ras-cAMP. These results suggest a model linking the Ras-cAMP pathway with regulation of G1 cyclin expression.A fundamental goal in biology is to understand how cells control proliferative growth. In recent years, progress toward this goal has been made in two important areas. The first area involves the identification and study of oncogenes-genes that in many cases encode proteins that carry signals regulating cellular proliferation. Mutations in these genes lead to aberrant signalling, unregulated proliferation, and cancer. The second area involves the discovery of two families of proteins, the cyclins and the cell cycle-dependent protein kinases (CDKs), that are believed to allow cells to pass checkpoints in the cell cycle (26,28). Included among these cell cycle checkpoints is the G1-to-S phase transition that is known as START in the yeast Saccharomyces cerevisiae or as the restriction point in mammals (16,25).In S. cerevisiae, three cyclin genes that affect passage through START have been identified: CLN1, CLN2, and CLN3. The protein products of these genes activate the only member of the CDK family known to exist in S. cerevisiae, encoded by CDC28. Activation of p34CDC28 enables cells to pass the START checkpoint. Cells remain viable after loss of any two of the G1 cyclin genes; however, loss of all three G1 cyclin genes leaves the cells arrested permanently at START, an effect similar to that produced by loss of the CDC28 kinase (29). In contrast, activating mutations in any of the G, cyclin genes, or the overexpression of any of these genes, results in small cells with an abbreviated or absent G, phase (8). These and other results suggest a pathway in which three redundant cyclins associate with and activate the p34CDC28 kinase in order to carry cells through START.Although the three mitotic cyclins appear to serve redundant functions, there are distinct differences between them. CLN1, CLN2, and CLN3 all show sequence homologies with the mitotic cyclins (7, 22, 32); however, CLNI and CLN2 show much greater similarity to each other than to CLN3. CLN3 also stands apart in its expression pattern. While CLNI and CLN2 expression peaks dramatically at the Ga/S boundary, the level of CLN3 message r...

Acetyl-CoA induces transcription of the key G1 cyclin CLN3 to promote entry into the cell division cycle in Saccharomyces cerevisiae

“…Our observations thus far are entirely consistent with previous studies showing that glucose is a potent inducer of CLN3 transcription (22). However, none of the well-known glucose or nutrientsignaling pathways, including cAMP-PKA, TOR, and various glucose transporters, seems to be required for glucose-induced CLN3 transcription (9,22,43). In contrast, the inhibition of glycolysis compromises glucose-induced CLN3 transcription, suggesting a downstream metabolite of glucose could be mediating its effects (9,22).…”

Ras Transformation Results in an Elevated Level of Cyclin D1 and Acceleration of G1 Progression in NIH 3T3 cells

“…In budding yeast cells, depending on the growth condition of this organism, activation of the Ras-cyclic AMP pathway could either activate or repress the expression of their G 1 cyclins. In the former case, this process is rapid and is independent of protein synthesis (14). In the latter case, activation of the same pathway would affect the critical size of cells before they divide (2,38).…”

Glucose Signaling in Saccharomyces cerevisiae

“…This requires sophisticated regulatory coordination between the metabolic engine of growth and the wide variety of structural events required for cellular reduplication. With respect to glucose signaling it has long seemed likely that there is a connection with the essential cyclin-dependent kinase Cdc28; the latter acts in concert with the G 1 cyclins Cln1 to -3 at the primary gating event (Start) in the cell division cycle (3,116,132,165,262,263,276,285,286,321,371,416,424; see references 83, 365, and 389 for reviews). Unfortunately many of the critical relationships between the growth and cell cycles have resisted elucidation.…”