Localization of the N-terminal and C-terminal ends of triadin with respect to the sarcoplasmic reticulum membrane of rabbit skeletal muscle

Antibodies were raised against synthetic peptides corresponding to the N-terminal (residues 2-17) and C-terminal (residues 691-706) ends of rabbit skeletal muscle triadin, a 95 kDa protein located in the sarcoplasmic reticulum membrane at the triad junction. The specificity of the antibodies generated was tested by ELISA and Western blot analysis. These tests demonstrated the ability of the antibodies to react specifically with the proteins. The anti-N-terminus antibodies bound to sarcoplasmic reticulum vesicles, indicating that the N-terminal end of the membrane-embedded triadin is exposed on the cytoplasmic side of the vesicles. In contrast, the anti-C-terminus antibodies were able to react with sarcoplasmic reticulum vesicles only after permeabilization of the vesicles with a detergent, indicating that the C-terminal end is exposed on the luminal side of the vesicles. These immunological data were complemented by proteolysis experiments using carboxypeptidases and endoproteinase Arg C. A mixture of carboxypeptidases A, B and Y was used to induce degradation of the C-terminal end of triadin in sarcoplasmic reticulum vesicles. This degradation, and a concomitant loss of reactivity of the anti-C-terminus antibodies in Western blots, was observed only when the vesicles were permeabilized, providing further evidence for the luminal localization of the C-terminal end of triadin. Treatment of sarcoplasmic reticulum vesicles with endoproteinase Arg C resulted in the removal of the N-terminal end of triadin, probably due to cleavage after Arg-34. This is a further indication of the cytoplasmic localization of the N-terminal end of triadin (and of its first 34 amino acids). When the proteolysis with endoproteinase Arg C was carried out with permeabilized vesicles, the cleavage occurred after Arg-141 or Arg-157, indicating that at least one of these residues is luminal.