THE SUBCELLULAR-LOCALIZATION OF E2F-4 IS CELL-CYCLE DEPENDENT

The E2F family of transcription factors plays a crucial role in cell c ycle progression, E2F activity is tightly regulated by a number of mec hanisms, which include the timely synthesis and degradation of E2F, in teraction with retinoblastoma protein family members (''pocket protein s''), association with DP heterodimeric partner proteins, and phosphor ylation of the E2F/DP complex, Here we report that another mechanism, subcellular localization, is important for the regulation of E2F activ ity. Unlike E2F-1, -2, or -3, which are constitutively nuclear, ectopi c E2F-4 and -5 were predominantly cytoplasmic, Cotransfection of expre ssion vectors encoding p107, p130, or DP-2, but not DP-1, resulted in the nuclear localization of E2F-4 and -5. Moreover, the transcriptiona l activity of E2F-4 was markedly enhanced when it was invariably nucle ar, Conversely, it was reduced when the protein was excluded from the nucleus, implying that E2F-4 transcription function depends upon its c ytological location, In keeping with this, the nuclear/cytoplasmic rat ios of endogenous E2F-4 changed as cells exited G(0), with high ratios in G(0) and early G(1) and a progressive increase in cytoplasmic E2F- 4 as cells approached S phase, Thus, the subcellular location of E2F-4 is regulated in a cell cycle-dependent manner, providing another pote ntial mechanism for its functional regulation.