A 107‐kD protein has been identified in primary cultures of chicken embryonic cardiomyocytes by immunoprecipitations with certain anti‐nebulin monoclonal antibodies (mAbs). These mAbs, prepared against a fragment of human skeletal muscle nebulin located near the carboxyl terminus, detect a 107‐kD protein in extracts of adult chicken heart, adult mouse heart, and adult rabbit heart by immunoblot analysis. A partial cDNA corresponding to this protein has been isolated by immunological screening of a chicken heart cDNA expression vector library. The partial cDNA encodes a 380‐amino acid open reading frame composed entirely of nebulin‐like 35‐residue modules marked by the highly conserved sequence motifs: SXXXYK and TPD. The open reading frame exhibits 60–85% homology with skeletal muscle nebulins from a variety of species. This cDNA recognizes an ˜8‐kb transcript in cardiac RNA and does not hybridize to skeletal muscle RNAs by northern analysis. Immunofluorescence localization of this nebulin‐like protein in primary cultures of chicken cardiomyocytes and embryonic chicken cardiac myofibrils indicates that the protein is localized to the I‐Z‐I complex of the myofibrils, extending approximately 25% of the thin filament length. Comparisons of the distribution of this protein relative to actin, myosin, and titin in spreading cardiomyocytes suggest that the cardiac nebulin‐like protein becomes aligned with the nascent myofibrils early during myofibrillogenesis. To distinguish this petite nebulin‐like protein from the 600–900 kD skeletal muscle nebulin, we have named it nebulette. © 1995 Wiley‐Liss, Inc.
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