Characterization and ultrastructural localization of a novel 90-kDa protein unique to skeletal muscle junctional sarcoplasmic reticulum.

Monoclonal antibodies were used to identify and characterize a novel 90 kDa protein that was specifically localized to the junctional sarcoplasmic reticulum of rabbit skeletal muscle. Biochemical experiments show that the 90 kDa protein is an integral membrane protein of the junctional face membrane and is a substrate for the intrinsic protein kinase in triads. Immunofluorescence staining of serial transverse sections of skeletal muscle with a monoclonal antibody to the 90 kDa protein showed preferential staining of type II "fast" fibers. Specific labeling was confined to the interphase between the A- and I-bands, where the triad structure is localized. Immunoelectron microscopical labeling further indicates that the 90 kDa protein, like the ryanodine receptor/Ca(2+)-release channel and triadin, is confined to the terminal cisternae of the sarcoplasmic reticulum. Western blot analysis with a combination of monoclonal antibodies against the 90 kDa protein shows that it is specifically expressed in skeletal muscle but not in cardiac muscle or brain. Similarly, specific immunofluorescence labeling to the 90 kDa protein was not detected in ventricular myocytes or vascular smooth muscle cells. The junctional localization and phosphorylation of this protein suggest that it may play an important regulatory or structural role in the skeletal muscle triad junction.