Figure 2

Model of cell cycle entry control by pocket protein. Beginning in the top left corner, quiescent cells repress transcription of E2F targets genes largely through the actions of p130. As cells progress into G1, complexes containing p107 and a repressor E2F such as E2F4 begin to replace p130. Furthermore, complexes of pRB and activator E2Fs such as E2F3 also become more abundant. Chromatin remodeling factors (CRF) are recruited to these complexes and mediate alterations to the chromatin environment, preventing transcription of E2F responsive genes. As a result, transcription of E2F target genes remains low until entry into S-phase. At the transition to S-phase, cyclin/CDK complexes phosphorylate the pocket proteins, dissociating them from the E2F/DP duplexes and transcription of E2F target genes proceeds through S phase. As part of this transition, the repressive heterochromatin changes that were present in G1 are reversed by the recruitment of new enzymes by the E2Fs, histone acetyltransferases (HAT) are examples of this type of enzyme. Another important change at the start of S-phase is the export of p130 and 107 proteins from the nucleus. At this point pocket proteins are thought to be relatively functionless until they are dephosphorylated and reactivated at the end of mitosis so that they can regulate transcription again during the next G1 phase.