Purification and Characterization of an α-Actinin-binding PDZ-LIM Protein That Is Up-regulated during Muscle Differentiation*

α-Actinin is required for the organization and function of the contractile machinery of muscle. In order to understand more precisely the molecular mechanisms by which α-actinin might contribute to the formation and maintenance of the contractile apparatus within muscle cells, we performed a screen to identify novel α-actinin binding partners present in chicken smooth muscle cells. In this paper, we report the identification, purification, and characterization of a 36-kDa smooth muscle protein (p36) that interacts with α-actinin. Using a variety of in vitrobinding assays, we demonstrate that the association between α-actinin and p36 is direct, specific, and saturable and exhibits a moderate affinity. Furthermore, native co-immunoprecipitation reveals that the two proteins are complexed in vivo. p36 is expressed in cardiac muscle and tissues enriched in smooth muscle. Interestingly, in skeletal muscle, a closely related protein of 40 kDa (p40) is detected. The expression of p36 and p40 is dramatically up-regulated during smooth and skeletal muscle differentiation, respectively, and p40 colocalizes with α-actinin at the Z-lines of differentiated myotubes. We have established the relationship between p36 and p40 by molecular cloning of cDNAs that encode both proteins and have determined that they are the products of a single gene. Both proteins display an identical N-terminal PDZ domain and an identical C-terminal LIM domain; an internal 63-amino acid sequence present in p36 is replaced by a unique 111-amino acid sequence in p40. Analysis of the sequences of p36 and p40 suggest that they are the avian forms of the actinin-associated LIM proteins (ALPs) recently described in rat (Xia, H., Winokur, S. T., Kuo, W.-L., Altherr, M. R., and Bredt, D. S. (1997) J. Cell Biol.139, 507–515). The expression of the human ALPgene has been postulated to be affected by mutations that cause facioscapulohumeral muscular dystrophy; thus, the characterization of ALP function may ultimately provide insight into the mechanism of this disease.