A Protein That Binds Specifically to the 1 Identified as the Muscle Form of Creatine

Published information on the properties of two proteins from chicken muscle, creatine kinase (MM- creatine kinase) and an M-line protein, suggested that they might be identical molecules. Different published pro- cedures were used to purify the two proteins to homo- geneity, and the properties of the two preparations were compared. Creatine kinase specific activity increased during purification of M-line protein, reaching a value comparable to that of purified MM-creatine kinase. The two proteins migrated identically in two electrophoretic systems and, after electrophoresis, both could be stained for creatine kinase activity. Double immunodiffusion tests with antibody prepared against MM-creatine kinase es- tablished the serological identity of the two protein prep- arations. Immunofluorescent studies showed that anti- serum against MM-creatine kinase was bound in a regular pattern at the centers of the A-band regions of isolated myofibrils. These data show conclusively that the M-line protein and MM-creatine kinase are identical.