BIOCHEMICAL-CHARACTERIZATION AND ULTRASTRUCTURAL-LOCALIZATION OF A MAJOR JUNCTIONAL SARCOPLASMIC-RETICULUM GLYCOPROTEIN (TRIADIN)

Monoclonal antibodies were used to identify a 94-kDa protein that was greatly enriched in triads and junctional face membranes (9.3 +/- 0.2% ) but not detected in the transverse tubular and nonjunctional sarcopl asmic reticulum membranes. The 94-kDa protein is a hydrophobic glycopr otein based on endoglycosidase H sensitivity, concanavalin A binding, and labeling with a hydrophobic probe. Sodium dodecyl sulfate-polyacry lamide gel electrophoresis in the absence and presence of reducing age nts suggests that this protein is present as a population of multimeri c structures containing a variable number of the 94-kDa subunits. Immu nofluorescent staining of serial transverse sections of skeletal muscl e shows staining of all fiber types with preferential staining of type II fast fibers. Specific immunofluorescence staining in longitudinal sections of skeletal muscle is confined to the interface between the A - and I-bands where the triad structures are localized. Immunocolloida l gold labeling revealed the 94-kDa glycoprotein to be localized over a region of the junctional sarcoplasmic reticulum where the ryanodine receptor/Ca2+ release channel is localized. The distribution and high abundance of the 94-kDa glycoprotein in the junctional membrane sugges t that it performs a structural or functional role in the storage or r elease of calcium from the junctional sarcoplasmic reticulum in skelet al muscle.