Cytoplasmic MLP function in myogenesis

Muscle LIM Protein (MLP) is a striated muscle-specific LIM domain containing protein that enhances myogenic differentiation and is critical to maintaining the structural integrity of the actin-based contractile apparatus. The ability of MLP to regulate myogenesis is particularly interesting since it exhibits multiple subcellular localizations, being found in both the nucleus and cytoplasm. Despite extensive biochemical analyses on MLP, the mechanism(s) by which it influences the myogenic program has remained largely undefined. In order to define the role of cytoplasmic MLP in muscle development and function, yeast two-hybrid screens were employed to identify cytoplasmic MLP binding partners. Several proteins were identified in this screen; one of which was the cytoskeletal protein βI-spectrin. Protein interaction assays confirmed that MLP and βI-spectrin associate with one another in vivo as well as when tested under several in vitro binding conditions. Subcellular localization studies also revealed that MLP co-localizes with β-spectrin at the sarcolemma overlying the Z- and M-lines of myofibrils in both cardiac and skeletal muscle tissue. Interestingly, β-spectrin is believed to play a key role in anchoring the contractile apparatus to the overlying membrane at subsarcolemmal regions known as costameres. By its association with β-spectrin, MLP appears to function as a key costamere protein, stabilizing the association of the contractile apparatus with the sarcolemma by linking the β-spectrin network to the α-actinin crosslinked actin filaments of the myofibril. This model is supported by the phenotype of MLP deficient mice that display severe disruptions of the cytoskeletal architecture (Arber et al., 1997). Further characterization of these mice indicates that MLP is critical for maintaining the organization of the β-spectrin lattice network. Examination of MLP in adult muscle suggests it may have a muscle fiber type specific role. MLP displays a differential gene expression pattern and subcellular localization in various fiber types, being found predominately in oxidative slow fibers. Finally, yeast two-hybrid screens lead to the identification of a novel MLP binding protein with similarity to D-lactate dehydrogenase enzyme. Identification of this new MLP binding partner may shed light on a novel function for MLP in striated muscle