T-system membranes in microsomal fractions of crayfish muscles: identification and differences in protein composition.

Specific binding of 3H-ouabain and ruthenium red (RR) to membranes of T-tubules in crayfish muscles was used to identify the subfraction containing vesicles originating from the T-system. The microsomal fraction was prepared by differential centrifugation, and subfractions were separated in continuous or discontinuous sucrose density gradients. 3H-ouabain binding was estimated by scintillation counting; RR binding was examined by electron microscopy. The light subfraction was identified using both methods as that containing vesicles of T-tubules. Protein separation by SDS-electrophoresis revealed marked differences between the subfraction containing vesicles of T-tubules and other subfractions, the most distinctive feature being the presence of a protein of Mr 46,000 predominantly in the light subfraction.