Multiple cyclin-dependent kinase complexes and phosphatases control G(2)/Mprogression in alfalfa cells

Reversible phosphorylation of proteins by kinases and phosphatases plays a key regulatory role in several eukaryotic cellular functions including the control of the division cycle. Increasing numbers of sequence and biochemic al data show the involvement of cyclin-dependent kinases (CDKs) and cyclins in regulation of the cell cycle progression in higher plants. The complexi ty represented by different types of CDKs and cyclins in a single species s uch as alfalfa, indicates that multicomponent regulatory pathways control G (2)/M transition. A set of cdc2-related genes (cdc2Ms A, B, D and F) was ex pressed in G(2) and M cells. Phosphorylation assays also revealed that at l east three kinase complexes (Cdc2Ms A/B, D and F) were successively active in G(2)/M cells after synchronization. Interaction between alfalfa mitotic cyclin (Medsa;CycB2;1) and a kinase partner has been reported previously. T he present yeast two-hybrid analyses showed differential interaction betwee n defined D-type cyclins and Cdc2Ms kinases functioning in G(2)/M phases. L ocalization of Cdc2Ms F kinase to the preprophase band (PPB), the perinucle ar ring in early prophase, the mitotic spindle and the phragmoplast indicat ed a pivotal role for this kinase in mitotic plant cells. So far limited re search efforts have been devoted to the functions of phosphatases in the co ntrol of plant cell division. A homologue of dual phosphatase, cdc25, has n ot been cloned yet from alfalfa; however tyrosine phosphorylation was indic ated in the case of Cdc2Ms A kinase and the p(13suc1)-bound kinase activity was increased by treatment of this complex with recombinant Drosophila Cdc 25. The potential role of serine/threonine phosphatases can be concluded fr om inhibitor studies based on okadaic acid or endothall. Endothall elevated the kinase activity of p13(suc1)-bound fractions in G(2)-phase alfalfa cel ls. These biochemical data are in accordance with observed cytological abno rmalities. The present overview with selected original data outlines a conc lusion that emphasizes the complexity of G(2)/M regulatory events in flower ing plants.