A novel mammalian protein, p55CDC, present in dividing cells is associated with protein kinase activity and has homology to the Saccharomyces cerevisiae cell division cycle proteins Cdc20 and Cdc4.

Abstract

A novel protein, p55CDC, has been identified in cycling mammalian cells. This transcript is readily detectable in all exponentially growing cell lines but disappears when cells are chemically induced to fall out of the cell cycle and differentiate. The p55CDC protein appears to be essential for cell division, since transfection of antisense p55CDC cDNA into CHO cells resulted in isolation of only those cells which exhibited a compensatory increase in p55CDC transcripts in the sense orientation. Immunoprecipitation of p5SCDC yielded protein complexes with kinase activity which fluctuated during the cell cycle. Since p55CDC does not have the conserved protein kinase domains, this activity must be due to one or more of the associated proteins in the immune complex. The highest levels of protein kinase activity were seen with a-casein and myelin basic protein as substrates and demonstrated a pattern of activity distinct from that described for the known cyclin-dependent cell division kinases. The p55CDC protein was also phosphorylated in dividing cells. The amino acid sequence of p55CDC contains seven repeats homologous to the ,( subunit of G proteins, and the highest degree of homology in these repeats was found with the Saccharomyces cerevisiae Cdc2O and Cdc4 proteins, which have been proposed to be involved in the formation of a functional bipolar mitotic spindle in yeast cells. The G,B repeat has been postulated to mediate protein-protein interactions and, in p55CDC, may modulate its association with a unique cell cycle protein kinase. These findings suggest that p55CDC is a component of the mammalian cell cycle mechanism.The eukaryotic cell cycle has a growth phase and a reproductive phase, the latter composed of the chromosome cycle and the centrosome cycle, which intersect in the establishment of the mitotic apparatus (for a review, see reference 47). The profound morphologic changes which result in mitosis are accompanied by a cascade of phosphorylation and dephosphorylation events. In mammalian cells, different complexes of kinases and their associated regulatory proteins control progression through discrete steps of the cell cycle (for a review, see references 59 and 68). While all eukaryotic cells use similar mechanisms to regulate progression through the stages of the cell cycle, it is clear that unique combinations of regulatory cyclins, kinases, and phosphatases are responsible for cell-and organism-specific patterns of cell division (16, 51, 52).A variety of kinases which control the crucial transitions through the cell cycle have been identified. The best characterized are the cyclin-dependent kinases, such as p34cdc2, which fluctuate in activity during the cell cycle (3, 18, 20, 28, 40-42, 48, 60, 78). The activity of many of these kinases is regulated by their association with one or more cyclins, which may be degraded at specific transition points in the cell cycle (44,49,59,68 499-2751. (MEK) kinases, which regulate MAP kinase activity (for a review, see reference 11). In addition, a novel...

p55CDC/hCDC20 is associated with BUBR1 and may be a downstream target of the spindle checkpoint kinase

“…Early studies have shown that p55CDC is associated with unknown protein kinase activities (Weinstein et al, 1994) and highly phosphorylated during the G 2 (Weinstein, 1997;Weinstein et al, 1994). We asked whether BUBR1 might be a candidate kinase that directly phosphorylated p55CDC.…”

“…It has been shown that CDC20 is a target of the yeast spindle checkpoint (Hwang et al, 1998). Human p55CDC is a phosphoprotein and the phosphorylation peaks at G 2 (Weinstein, 1997;Weinstein et al, 1994). The phosphorylation drops signi®cantly as the cell enters mitosis (Weinstein, 1997).…”

“…Recently, Farruggio et al have shown that aurora2/Aik kinase associates with Cdc20/p55CDC in human somatic cells and that Cdc20/p55CDC-associated aurora kinase activity peaks at G2 phase of the cell cycle (Farruggio et al, 1999). It is possible that protein kinases other than BUBR1 are also involved in regulating p55CDC activity because early studies have demonstrated that p55CDC is associated with more than one protein kinase activities (Weinstein, 1997;Weinstein et al, 1994).…”

“…Recently, it has been shown that MAD2 controls the activity of CDC20/Slp1, a WD repeat protein required for exit from mitosis (Hwang et al, 1998). p55CDC is a mammalian homolog of CDC20 of the budding yeast (Weinstein, 1997;Weinstein et al, 1994). The p55CDC protein mediates association of MAD2 to APC and is required for the metaphase and anaphase transition (Kallio et al, 1998).…”

“…It is also implicated in regulation of apoptosis in myeloid cells (Kao et al, 1996). Early studies have shown that high levels of p55CDC are detected in proliferating cells and that it is associated with protein kinase activities (Weinstein et al, 1994). Micro-injection of an anti-p55CDC antibody into mammalian cells either blocks cells at metaphase or impairs late mitotic events (Kallio et al, 1998).…”