DESCRIPTION: This is a proposal by an experienced investigator to study the molecular mechanism by which rhabdomyosarcoma tumor cells inhibit myogenic differentiation. Several protooncogenes were found to be amplified in these cells, but only one, MDM2, was found to inhibit myogenesis after transfection into normal muscle cells. Such observations, and an impressive amount of preliminary results, lead to the central hypothesis that MDM2 blocks myogenic differentiation by binding to the transcriptional activator MEF2 and thereby inhibiting its cooperative interactions with another muscle-specific transcriptional activator, MyoD. This proposal will test that hypothesis by (1) characterizing the MDM2-MEF2 binding interaction both in vitro and in vivo; (2) determination of the role of that binding interaction in myogenic differentiation by loss and gain of function experiments; and (3) analysis of a proposed MDM2-pRB interaction as a mechanism for displacing MEF2 from binding pRB and thereby activating the myogenic differentiation program.