The localization of human cyclins B1 and B2 determines CDK1 substrate specificity and neither enzyme requires MEK to disassemble the Golgi apparatus

In this paper. we show that substrate specificity is primarily conferred on human mitotic cyclin-dependent kinases (CDKs) by their subcellular localiz ation. The difference in localization of the B-type cyclin-CDKs underlies t he ability of cyclin B1-CDK1 to cause chromosome condensation, reorganizati on of the microtubules, and disassembly of the nuclear lamina and of the Go lgi apparatus, while it restricts cyclin B2-CDK1 to disassembly of the Golg i apparatus. We identify the region of cyclin B2 responsible for its locali zation and show that this will direct cyclin B1 to the Golgi apparatus and confer upon it the more limited properties of cyclin B2. Equally, directing cyclin B2 to the cytoplasm with the NH2 terminus of cyclin B1 confers the broader properties of cyclin B1. Furthermore. we show that the disassembly of the Golgi apparatus initiated by either mitotic cyclin-CDK complex does not require mitogen-activated protein kinase kinase (MEK) activity.