C. Robinson et al., CELL-CYCLE REGULATION OF B-MYB PROTEIN EXPRESSION - SPECIFIC PHOSPHORYLATION DURING THE S-PHASE OF THE CELL-CYCLE, Oncogene, 12(9), 1996, pp. 1855-1864
Previous studies revealed that transcription of B-Myb, which encodes a
transcription factor related to the c-Myb proto-oncoprotein, is cell-
cycle regulated by an E2F transcription factor-mediated repression mec
hanism operating in G(0)/G(1). To begin to determine the consequences
of transcriptional regulation on B-Myb function, we report here furthe
r studies of B-Myb protein expression in the cell cycle. We found that
G(0)-arrest of serum-deprived mouse fibroblasts was achieved without
significant reduction in B-Myb levels, moreover, overexpression of B-M
yb in stably transfected cells did not prevent their entry into G(0).
Following serum-induction of arrested fibroblasts, B-Myb abundance inc
reased as cells entered S phase to levels significantly greater than f
ound in cycling cells. This was accompanied by the appearance of a nov
el phosphorylated form of B-Myb (112 kDa) of distinctly lower electrop
horetic mobility than B-Myb present in G(1) (110 kDa). The 112 kDa spe
cies was S phase-specific even in transfected cells overexpressing B-M
yb. Consistent with modification in the S phase of the cell cycle, pre
liminary evidence suggested that a cyclin A/cdk2, but not cyclin E/cdk
2 or cyclin D1/cdk4, complex could induce a similar electrophoretic mo
bility change in baculovirus-specified B-Myb. These findings show that
B-Myb expression may be subject to two levels of control during the c
ell cycle, transcription and protein phosphorylation.