There are 2 projects underway in the laboratory. One to understand how proteolysis controls the abundance of p27, and the other to understand how translational regulatory signals control p27 abundance. Prior work from our laboratory and others indicated that small changes in p27 abundance (2-fold) can have dramatic biological consequences vis-a-vis proliferation or growth arrest. We demonstrated, in agreement with others, that as cells enter S-phase, p27 is degraded; and this is dependent on the ubiquitin-dependent proteasome and cdk2 activity.
We are now defining what proteins participate in p27 degradation by fractionating these extracts and reconstituting in vitro the degradation pathway. The essential question remains however, why is p27 degraded as cells commit to the cell cycle? In the yeast S. cervisiae, it is clear that following commitment and before DNA replication, an essential ubiquitin-dependent proteolytic event occurs (the degradation of another cdk inhibitor, sic1). However, this machinery has more than sic1 as a target -- indeed, one is another cdk inhibitor, Far1, which is used to mediate growth arrest in response to the environmental signal of mating pheromone.
Thus, this single event following commitment, ubiquitin-dependent degradation, has 2 functions: 1 in promoting S-phase entry, and the other in eliminating the ability of a cell to respond to environmental signals that regulate growth arrest. We are determining which of these functions is important for p27 degradation by using a combined cell biology and mouse genetics approach.
