Nesprin-1K K is a spectrin repeat (SR)-containing, transmembrane protein of the inner nuclear membrane, and is highly expressed in muscle cells. A yeast two-hybrid screen for nesprin-1K K-interacting proteins showed that nesprin-1K K interacted with itself. Blot overlay experiments revealed that nesprin-1K K's third SR binds the ¢fth SR. The carboxy-terminal half of nesprin-1K K directly bound lamin A, a nuclear intermediate ¢lament protein. Biochemical analysis demonstrated that nesprin-1K K dimers bind directly to the nucleoplasmic domain of emerin, an inner nuclear membrane protein, with an a⁄nity of 4 nM. Binding was optimal for full nucleoplasmic dimers of nesprin-1K K, since nesprin fragments SR1-5 and SR5-7 bound emerin as monomers with a⁄nities of 53 nM and 250 mM, respectively. We propose that membrane-anchored nesprin-1K K antiparallel dimers interact with both emerin and lamin A to provide sca¡olding at the inner nuclear membrane. ß 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.

Mutations in the genes encoding the inner nuclear membrane proteins lamin A/C and emerin produce cardiomyopathy and muscular dystrophy in humans and mice. The mechanism by which these broadly expressed gene products result in tissue-specific dysfunction is not known. We have identified a protein of the inner nuclear membrane that is highly expressed in striated and smooth muscle. This protein, myne-1 (myocyte nuclear envelope), is predicted to have seven spectrin repeats, an interrupted LEM domain and a single transmembrane domain at its C-terminus. We found that myne-1 is expressed upon early muscle differentiation in multiple intranuclear foci concomitant with lamin A/C expression. In mature muscle, myne-1 and lamin A/C are perfectly colocalized, although colocalization with emerin is only partial. Moreover, we show that myne-1 and lamin A/C coimmunoprecipitate from differentiated muscle in vitro. The muscle-specific inner nuclear envelope expression of myne-1, along with its interaction with lamin A/C, indicates that this gene is a potential mediator of cardiomyopathy and muscular dystrophy.