Cell cycle regulation in Aspergillus by two protein kinases.

Great progress has recently been made in our understanding of the regulation of the eukaryotic cell cycle, and the central role of cyclin-dependent kinases is now clear. In Aspergillus nidulans it has been established that a second class of cell-cycle-regulated protein kinases, typified by NIMA (encoded by the nimA gene), is also required for cell cycle progression into mitosis. Indeed, both p34cdc2/cyclin B and NIMA have to be correctly activated before mitosis can be initiated in this species, and p34cdc2/cyclin B plays a role in the mitosis-specific activation of NIMA. In addition, both kinases have to be proteolytically destroyed before mitosis can be completed. NIMA-related kinases may also regulate the cell cycle in other eukaryotes, as expression of NIMA can promote mitotic events in yeast, frog or human cells. Moreover, dominant-negative versions of NIMA can adversely affect the progression of human cells into mitosis, as they do in A. nidulans. The ability of NIMA to influence mitotic regulation in human and frog cells strongly suggests the existence of a NIMA pathway of mitotic regulation in higher eukaryotes. A growing number of NIMA-related kinases have been isolated from organisms ranging from fungi to humans, and some of these kinases are also cell-cycle-regulated. How NIMA-related kinases and cyclin-dependent kinases act in concert to promote cell cycle transitions is just beginning to be understood. This understanding is the key to a full knowledge of cell cycle regulation.