The p130 pocket protein: keeping order at cell cycle exit/re-entrance transitions.

Pocket proteins, including the retinoblastoma susceptibility gene product (pRB) and the related proteins p107 and p130, function at cell cycle regulatory steps that link cyclin/CDK-integrated positive and negative growth signals with E2F transcription factor activity on genes required for cell cycle progression. Protein complex formation between pocket proteins and members of the E2F family of transcription factors determines whether E2F complexes act as transcriptional activators or repressors. Experimental work over the last few years indicates that individual pocket proteins interact with specific E2F members to regulate the transcription of certain genes under diverse cell growth conditions. Among these protein associations, p130-containing E2F complexes seem to be of particular importance in controlling gene transcription in quiescent and differentiating cells by repressing the transcription of a set of E2F-responsive genes. Once the cells are progressing through the G1 phase of the cell cycle, pocket protein-mediated regulation of E2F activity is assumed by pRB and p107. p130-mediated transcriptional regulation thus seems to prevent a gene expression program characteristic of dividing cells at the cell cycle exit and re-entrance transitions and in quiescent cells.