FIG. 9.

Cooperative effects of the cyclin A CRE and CDE sites. Activation of a receptor tyrosine kinase (RTK) by growth factor (GF) leads to the phosphorylation of CREB in early G₁ phase, an effect that is permissive but not sufficient for induction of the cyclin A promoter. Receptor tyrosine kinases also cooperate with the organized actin cytoskeleton to sustain ERK activity and to regulate cyclin D1 expression, pocket protein phosphorylation, and occupancy of the CDE site. This effect is important for the timing of cyclin A promoter activity.