Nuclear accumulation of the E2F heterodimer regulated by subunit composition and alternative splicing of a nuclear localization signal.

The cellular transcription factor E2F plays a critical role in integrating cell cycle progression with the transcription apparatus by virtue of a physical interaction and control by key regulators of the cell cycle, such as pRb, cyclins and cyclin-dependent kinases. Generic E2F DNA binding activity arises when a member of two families of proteins, E2F and DP, form heterodimeric complexes, an interaction which results in co-operative transcriptional and DNA binding activity. Here, we characterise a new and hitherto unexpected mechanism of control influencing the activity of E2F which is mediated at the level of intracellular location through a dependence on heterodimer formation for nuclear translocation. Nuclear accumulation is dramatically influenced by two distinct processes: alternative splicing of a nuclear localization signal and subunit composition of the E2F heterodimer. These data define a new level of control in the E2F transcription factor whereby interplay between subunits dictates the levels of nuclear DNA binding activity.