I-protein is localized at the junctional region of A-bands and I-bands of chicken fresh myofibrils.

FITC-labeled antibodies raised against chicken myofibrillar I-protein stained chicken myofibrils, which were fixed with formalin immediately after being cut from the sacrificed chicken breast muscle, at the junctional region of A-bands and I-bands. On the other hand, the antibodies stained the glycerinated myofibrils at the region around Z-bands. Aged glycerinated myofibrils stored in a cold room became stained with the same antibodies at the M-line and the A-band region except for the H-zone and the Z-band. I-Protein, which was originally localized at the A-I junctions, moved to the region around Z-bands and A-bands during the process of preparing myofibrils, paralleling the deterioration of myofibrils. Although I-protein is easily released from its original position, it is not a cytoplasmic protein of muscle but an intrinsic myofibrillar component, because immunoblotting tests showed that I-protein is contained in the myofibrillar fraction and not in the muscular cytoplasmic fraction.