Muscle lim protein (MLP) has emerged as a critical regulator of striated muscle 
physiology and pathophysiology. Mutations in cysteine and glycine-rich protein 3 
(CSRP3), the gene encoding MLP, have been directly associated with human 
cardiomyopathies, whereas aberrant expression patterns are reported in human 
cardiac and skeletal muscle diseases. Increasing evidence suggests that MLP has 
an important role in both myogenic differentiation and myocyte cytoarchitecture, 
although the full spectrum of its intracellular roles has not been delineated. 
We report the discovery of an alternative splice variant of MLP, designated as 
MLP-b, showing distinct expression in neuromuscular disease and direct roles in 
actin dynamics and muscle differentiation. This novel isoform originates by 
alternative splicing of exons 3 and 4. At the protein level, it contains the 
N-terminus first half LIM domain of MLP and a unique sequence of 22 amino acids. 
Physiologically, it is expressed during early differentiation, whereas its 
overexpression reduces C2C12 differentiation and myotube formation. This may be 
mediated through its inhibition of MLP/cofilin-2-mediated F-actin dynamics. In 
differentiated striated muscles, MLP-b localizes to the sarcomeres and binds 
directly to Z-disc components, including α-actinin, T-cap and MLP. The findings 
of the present study unveil a novel player in muscle physiology and 
pathophysiology that is implicated in myogenesis as a negative regulator of 
myotube formation, as well as in differentiated striated muscles as a 
contributor to sarcomeric integrity.