The cDNA of a novel protein, which contains the association domain of α isoform of calmodulin-dependent protein kinase II (CaM-kinase Ilα), was cloned from rat skeletal muscle. This protein, called αKAP, consisted of 200 amino acid residues with a molecular weight of 22, 583. αKAP has a highly hydrophobic amino-terminal stretch of 25 amino acids which is absent from CaM-kinase IIα, suggesting that this protein is either a secretory protein or an integral membrane protein. Northern blot analysis with a probe specific for αKAP detected three distinct mRNA species of 4.0, 2.4, and 1.5 kb in rat skeletal muscle. The 4.0- and 2.4-kb RNAs were also detected in heart, and at much lower levels in lung, kidney, and testis. Western blot analysis, using antibody raised against a synthetic peptide corresponding to the carboxyl-terminal 15 amino acids, revealed a single band corresponding in mobility to a molecular weight of 21, 000 in crude extracts of both rat skeletal muscle and bacteria transformed with the cDNA, suggesting that no significant post-translational modification, such as excision of the amino-terminal hydrophobic segment, occurred. This, together with the fact that αKAP was recovered in the high-speed pellet in skeletal muscle, indicated that this protein may be an integral membrane protein.