A novel Rb- and p300-binding protein inhibits transactivation by MyoD

The retinoblastoma protein (Rb) regulates both the cell cycle and tissue-sp ecific transcription, by modulating the activity of factors that associate with its A-B and C pockets. In skeletal muscle, Rb has been reported to reg ulate irreversible cell cycle exit and muscle-specific transcription. To id entify factors interacting with Rb in muscle cells, we utilized the yeast t wo-hybrid system, using the A-B and C pockets of Rb as bait. A novel protei n we have designated EIA-like inhibitor of differentiation 1 (EID-1), was t he predominant Rb-binding clone isolated. It is preferentially expressed in adult cardiac and skeletal muscle and encodes a 187-amino-acid protein, wi th a classic Rb-binding motif (LXCXE) in its C terminus. Overexpression of EID-1 in skeletal muscle inhibited tissue-specific transcription. Repressio n of skeletal muscle-restricted genes was mediated by a block to transactiv ation by MyoD independent of G(1) exit and, surprisingly, was potentiated b y a mutation that prevents EID-1 binding to Rb. Inhibition of MyoD may be e xplained by EID-1's ability to bind and inhibit p300's histone acetylase ac tivity, an essential MyoD coactivator. Thus, EID-1 binds both Rb and p300 a nd is a novel repressor of MyoD function.