Role of the fission yeast nim 1 protein kinase in the cell cycle response to nutritional signals.

The fission yeast cdr1/nim1 protein kinase phosphorylates and inactivates the weel cdc2-inhibitory kinase. We have investigated the role played by cdr1/nim1 in the connection between nutritional signals and the cell cycle machinery. We show that loss of nim1 activity impairs the appropriate cellular adaptation to nutritional changes. However, the reduction in cell size at division in response to nitrogen starvation is independent of nim1. Moreover, we report that nim1 is an unstable protein that is rapidly degraded upon starvation, through a mechanism that is dependent upon protein synthesis. We propose that nim1, as a constitutive indirect activator of cdc2 at mitosis, favors the cellular response to starvation but does not actively participate in it. On the contrary, upon nitrogen starvation nim1 must be actively destroyed to protect the cells from a commitment into the cell cycle under unfavourable growth conditions.