REGULATION OF G(1) PROGRESSION BY E2A AND ID HELIX-LOOP-HELIX PROTEINS

In NIH3T3 fibroblasts, the ubiquitous helix-loop-helix (HLH) protein E2A (E12/E47) and the myogenic HLH proteins MyoD, MRF4 and myogenin are growth-inhibitory, while two ubiquitous Id proteins lacking the basic region are not. The dimerization domain mediates inhibition. However, in addition to the HLH region, E2A contains two inhibitory regions overlapping with the main transcriptional activation domains. The growth-suppressive activity of the intact E47 as well as MyoD was counteracted bp the Id proteins. When E47 lacking the HLH domain was overexpressed, Id could no longer reverse growth inhibition. By increasing the amount of E47 with an inducible system or neutralizing the endogenous Id with microinjected anti-Id antibodies, withdrawal from the cell cycle occurred within hours before the G(1)-S transition point. The combined results suggest that the Id proteins are required for G(1) progression. The antagonism between the E2A and Id proteins further suggests that both are involved in regulatory events prior to or near the restriction point in the G(1) phase of the cell cycle.