In yeast cells that have not been exposed to pheromone, the majority of Ste5, a scaffold protein for mitogen-activated protein kinases (MAPKs), is located in the nucleus. Pheromone stimulation promotes nuclear export of Ste5 into the cytoplasm, where it is subsequently recruited to the plasma membrane with associated MAPKs, causing amplification of MAPK signaling. The role of the nuclear pool of Ste5 in the pheromone response pathway has been unclear. Finding that pheromone exposure increased the stability of Ste5 tagged with green fluorescent protein (GFP-Ste5), Garrenton et al. surmised that the nuclear pool of Ste5 may be subject to degradation. They found that GFP-Ste5 was less stable in a cell line lacking the nuclear exportin Msn5, whereas Ste5 stability was increased in cells in which nuclear import had been blocked, such as through inhibition of Ran GTPases involved in nucleocytoplasmic trafficking of proteins. Degradation of Myc-tagged Ste5 occurred through the proteasome and required a nuclear Skp1-Cullin/Cdc53-F box protein (SCF) ubiquitin E3 complex that contained the substrate recognition factor Cdc4 (SCFCdc4). As with other substrates of SCFCdc4, proteasomal degradation of endogenous Ste5 was enhanced by its phosphorylation by the major G1 cyclin-dependent kinase (CDK) in yeast. CDK activity increases as cells commit to the cell cycle and, as would be expected, the amount of GFP-Ste5 was highest in cells poised to enter the cell cycle (i.e., cells in G1) and decreased as cells progressed through the cell cycle. The authors proposed that the degradation of Ste5 in the nucleus may limit excess MAPK pathway activation and that CDK phosphorylation of Ste5 may function to inhibit the pheromone response pathway in actively dividing cells.
L. S. Garrenton, A. Braunwarth, S. Irniger, E. Hurt, M. Künzler, J. Thorner, Nucleus-specific and cell cycle-regulated degradation of mitogen-activated protein kinase scaffold protein Ste5 contributes to the control of signaling competence. Mol. Cell. Biol. 29, 582–601 (2009). [Abstract] [Full Text]