MUSCLE LIM PROTEIN PROMOTES MYOGENESIS BY ENHANCING THE ACTIVITY OF MYOD

The muscle LIM protein (MLP) is a muscle-specific LIM-only factor that exhibits a dual subcellular localization, being present in both the n ucleus and in the cytoplasm, Overexpression of MLP in C2C12 myoblasts enhances skeletal myogenesis, whereas inhibition of MLP activity block s terminal differentiation. Thus, MLP functions as a positive developm ental regulator, although the mechanism through which MLP promotes ter minal differentiation events remains unknown. While examining the dist inct roles associated with the nuclear and cytoplasmic forms of MLP, w e found that nuclear MLP functions through a physical interaction with the muscle basic helix-loop-helix (bHLH) transcription factors MyoD, MRF4, and myogenin, This interaction is highly specific since MLP does not associate with nonmuscle bHLH proteins E12 or E47 or with the myo cyte enhancer factor-2 (MEF2) protein, which acts cooperatively with t he myogenic bHLH proteins to promote myogenesis, The first LIM motif i n MLP and the highly conserved bHLH region of MyoD are responsible for mediating the association between these muscle-specific factors. MLP also interacts with MyoD-E47 heterodimers, leading to an increase in t he DNA-binding activity associated with this active bHLH complex. Alth ough MLP lacks a functional transcription activation domain, we propos e that it serves as a cofactor for the myogenic bHLH proteins by incre asing their interaction with specific DNA regulatory elements, Thus, t he functional complex of MLP-MyoD-E protein reveals a novel mechanism for both initiating and maintaining the myogenic program and suggests a global strategy for how LIM-only proteins may control a variety of d evelopmental pathways.