ROLE OF THE FISSION YEAST NIM1 PROTEIN-KINASE IN THE CELL-CYCLE RESPONSE TO NUTRITIONAL SIGNALS

The fission yeast cdr1/nim1 protein kinase phosphorylates and inactiva tes the wee1 cdc2-inhibitory kinase. We have investigated the role pla yed 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 r eduction in cell size at division in response to nitrogen starvation i s independent of nim1. Moreover, we report that nim1 is an unstable pr otein that is rapidly degraded upon starvation, through a mechanism th at is dependent upon protein synthesis. We propose that nim1, as a con stitutive indirect activator of cdc2 at mitosis, favors the cellular r esponse 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 unfavour able growth conditions. (C) 1997 Academic Press.