Cypher, a striated muscle-restricted PDZ and LIM domain-containing protein, binds to alpha-actinin-2 and protein kinase C

We have cloned and characterized a novel striated muscle-restricted protein (Cypher) that has two mRNA splice variants, designated Cypher1 and Cypher2 . Both proteins contain an amino-terminal PDZ domain. Cypher1, but not Cyph er2, contains three carboxyl-terminal LIM domains and an amino acid repeat sequence that exhibits homology to a repeat sequence found in the largest s ubunit of RNA polymerase II. cypher1 and cypher2 mRNAs exhibited identical expression patterns. Both are exclusively expressed in cardiac and striated muscle in embryonic and adult stages. By biochemical assays, we have demon strated that Cypher1 and Cypher2 bind to a-actinin-a via their PDZ domains. This interaction has been further confirmed by immunohistochemical studies that demonstrated co-localization of Cypher and a-actinin at the Z-lines o f cardiac muscle. We have also found that Cypher1 binds to protein kinase C through its LIM domains. Phosphorylation of Cypher by protein kinase C has demonstrated the functional significance of this interaction. Together, ou r data suggest that Cypher1 may function as an adaptor in striated muscle t o couple protein kinase C-mediated signaling, via its LIM domains, to the c ytoskeleton (cu-actinin-a) through its PDZ domain.