Cyclin/Cdk complexes: their involvement in cell cycle progression and mitotic division

DNA replication and mitosis are dependent on the activity of cyclin-depende nt protein kinase (CDK) enzymes, which are heterodimers of a catalytic subu nit with a cyclin subunit. Cyclin binding to specific individual proteins i s thought to provide potential substrates to Cdk. Protein binding by cyclin s is assessed in terms of its mechanisms and biological significance, using evidence from diverse organisms including substrate specificity in animal Cdk enzymes containing D-, A-, and B-type cyclins and extensive cyclin gene manipulations in yeasts. Assembly of protein complexes with cyclin/Cdk is noted and the capacity of the cyclin-dependent kinase subunit Cks. in such complex, to extend the range of Cdk substrates is documented and discussed in terms of cell cycle regulation. Cell cycle progression involves changing abundance of individual cyclins, due to changing rates of their transcript ion or proteolysis, with consequent changes in the substrates of CDK throug h the cell cycle. Some overlap of the functions of individual cyclins in vi vo has been identified hy cyclin deletions and is suggested to follow a pat tern in which cyclins can commonly complete functions initiated by the prec eding cyclins well enough to preserve viability as groups of cyclins are re moved by proteolysis. Cyclin accumulation is particularly important in term inating the GI phase, M when it raises CDK activity and starts events leadi ng to DNA replication. It is suggested that plants share this mechanism. Th e distribution of cyclins and Cdk in maize root tip cells during mitosis an d cytokinesis indicates the presence of Cdk1 (Cdc2a) and cyclin CycBlzm;2 a t the mature and disassembling preprophase band and the presence of CycBlzm ;2 at condensing and condensed chromosomes. Both observations correlate wit h the earlier-reported capacity of injected metaphase cyclin/CDK to acceler ate preprophase hand disassembly and chromosome condensation and with obser vations of the location of Cdk and cyclins in other laboratories. Additiona lly CycBlzm;2 is seen at the nuclear envelope during its breakdown, which c orrelates with an acceleration of the process hy injected metaphase cyclin B/CDK. A phenomenon possibly unique to the plant kingdom is the persistence of mitotic cyclins after anaphase. Participation of cyclins in cytokinesis is indicated by the concentration of the mitotic cyclin CycA1;zm;1 at the phragmoplast. It is suggested that cyclins have a general function of spati ally focusing Cdk activity and that in the plant cell the concentrations of cyclins are important mediators of CDK activity at the cytoskeleton, chrom osomes, spindle, nuclear envelope, and phragmoplast.