Isolation and characterization of a new 40-kilodalton protein from bovine cardiac muscle.

A new protein having a subunit weight of 40,000 has been purified from myosin-extracted bovine cardiac myofibrils. Its amino acid composition and isoelectric point are distinct from actin, eu-actinin, and a variety of sarcoplasmic proteins of similar size. Affinity-purified antibodies made to this protein only react with a single 40-kDa protein band from cardiac myofibrils on immunoblots. The anti-40-kDa protein also shows cross-reactivities with cardiac myofibrils from rabbits, rats, and chickens. Immunofluorescence studies demonstrate that the 40-kDa protein is localized at the Z-bands of cardiac myofibrils and at the intercalated discs. The antibody did not react with skeletal muscle myofibrils by immunofluorescence or immunoblotting. It appears that the 40-kDa protein may play a role in the strong attachments between adjacent myofibrils in cardiac muscle.