E2F ACTIVITY IS REGULATED BY CELL CYCLE-DEPENDENT CHANGES IN SUBCELLULAR-LOCALIZATION

E2F directs the cell cycle-dependent: expression of genes that induce or regulate the cell division process, In mammalian cells, I-his trans criptional activity arises from the combined properties of multiple E2 F-DP heterodimers. In this study, we show that the transcriptional pot ential of individual E2F species is dependent upon their nuclear local ization. This is a constitutive property of E2F-1, -2, and -3, whereas the nuclear localization of E2F-4 is dependent upon its association w ith other nuclear factors, We previously showed that E2F-4 accounts fo r the majority of endogenous E2F species. We now show that the subcell ular localization of E2F-4 is regulated in a cell cycle-dependent mann er that results in the differential compartmentalization of the variou s E2F complexes. Consequently, in cycling cells, the majority of the p 107-E2F, p130-E2F, and free E2F complexes remain in the cytoplasm, In contrast, almost ail of the nuclear E2F activity is generated by pRB-E 2F. This complex is present at high levels during G(1) but disappears once the cells have passed the restriction point. Surprisingly, dissoc iation of this complex causes little increase in the levels of unclear free E2F activity. This observation suggests that the repressive prop erties of the pRB-E2F complex play a critical role in establishing the temporal regulation of E2F-responsive genes, Bow the differential sub cellular localization of pRB, p107, and p130 contributes to their diff erent biological properties is also discussed.