SPATIAL-ORGANIZATION OF THE NIM1-WEE1-CDC2 MITOTIC CONTROL NETWORK INSCHIZOSACCHAROMYCES-POMBE

In Schizosaccharomyces pombe the onset of mitosis is regulated by a ne twork of protein kinases and phosphatases. The M-phase inducing Cdc2-C dc13 cyclin-dependent kinase is inhibited by Wee1 tyrosine kinase and activated by Cdc25 phosphatase. Wee1 is negatively regulated by Nim1 p rotein kinase. Here, we describe investigations aimed at better unders tanding the role of Nim1 in the mitotic control. The most important fi nding to emerge from these studies is that Wee1 and Nim1 have differen t patterns of intracellular localization. Immunofluorescence confocal microscopy has revealed that Nim1 is localized in the cytoplasm, where as its substrate Wee1 is predominantly localized in the nucleus. Previ ous studies showed that the Cdc2-Cdc13 complex is located in the nucle us. Diversion of Nim1 to the nucleus, accomplished by addition of the SV40 nuclear localization signal, caused the advancement of M, confirm ing that Nim1 has restricted access to Wee1 in vivo. We propose that t he intracellular distribution of Nim1 and Wee1 may serve to coordinate the regulation of nuclear Cdc2-Cdc13 with cytoplasmic growth.