CBP/p300 histone acetyl-transferase activity is important for the G1/S transition

Transforming viral proteins such as E1A which force quiescent cells into S phase have two essential cellular target proteins, Rb and CBP/p300, Rb regu lates the G1/S transition by controlling the transcription factor E2F, CBP/ p300 is a transcriptional co-activator with intrinsic histone acetyl-transf erase activity. This activity is regulated in a cell cycle dependent manner and shows a peak at the G1/S transition, suggesting a function for CBP/p30 0 in this crucial step of the cell cycle. Here, we have artificially modula ted CBP/p300 levels in individual cells through microinjection of specific antibodies and expression vectors. We show that CBP/p300 is required for ce ll proliferation and has an essential function during the G1/S transition. Using the same microinjection system and GFP-reporter vectors, we demonstra te that CBP/p300 is essential for the activity of E2F, a transcription fact or that controls the G1/S transition. In addition, our results suggest that CBP HAT activity is required both for the G1/S transition and for E2F acti vity. Thus CBP/p300 seems to be a versatile protein involved in opposing ce llular processes, which raises the question of how its multiple activities are regulated.