Degradation of B-Myb by ubiquitin-mediated proteolysis: involvement of theCdc34-SCFp45Skp2 pathway

B-Myb, a highly conserved member of the Myb oncoprotein family, is a 110 kD a sequence-specific DNA binding protein expressed in virtually all prolifer ating cells, B-myb expression reaches its maximum at the G1/S phase boundar y and during the S phase of the cell cycle. We have previously shown that B -Myb activity is cell cycle regulated and it is controlled by the antagonis tic effects of cyclin Dt and A. Here we show that ectopic expression of cyc lin A causes a pronounced reduction of B-Myb protein level, We provide evid ence that in addition to triggering B-Myb activity an important effect of c yclin A is to facilitate multiple ubiquitination of B-Myb, The C-terminal d omain of B-Myb is of key importance in mediating this effect of cyclin A, C ontrary to full-length B-Myb, a C-terminal deletion mutant displays activit y irrespective of cyclin A expression, does not undergo ubiquitination, and its half-life is not affected by cyclin A. Ectopic expression of either Cd c34 or the F-box protein p45(Skp2), respectively the E2 and E3 components o f a ubiquitination pathway that regulates the G1/S transition, accelerates degradation of B-Myb, We show that B-Myb physically and functionally intera cts with components of the Cdc34-SCFp45Skp2 ubiquitin pathway and propose t hat B-Myb degradation may be required for controlling the correct alternati on of events during progression through the cell division cycle.