BIOCHEMICAL-CHARACTERIZATION AND MOLECULAR-CLONING OF CARDIAC TRIADIN

Triadin is an intrinsic membrane protein first identified in the skele tal muscle junctional sarcoplasmic reticulum and is considered to play an important role in excitation-contraction coupling, Using polyclona l antibodies to skeletal muscle triadin, me have identified and charac terized three isoforms in rabbit cardiac muscle. The cDNAs encoding th ese three isoforms of triadin have been isolated by reverse transcript ion-polymerase chain reaction and cDNA library screening. The deduced amino acid sequences show that these proteins are identical in their N -terminal sequences, whereas the C-terminal sequences are distinct fro m each other and from that of skeletal muscle triadin. Based upon both the amino acid sequences and biochemical analysis, all three triadin isoforms share similar membrane topology with skeletal muscle triadin. Immunofluorescence staining of rabbit cardiac muscle with antibodies purified from the homologous region of triadin shows that cardiac tria din is primarily confined to the I-band region of cardiac myocytes, wh ere the junctional and corbular sarcoplasmic reticulum is located, Fur thermore, we demonstrate that the conserved region of the luminal doma in of triadin is able to bind both the ryanodine receptor and calseque strin in cardiac muscle. These results suggest that triadin colocalize s with and binds to the ryanodine receptor and calsequestrin and carri es out a function in the lumen of the junctional sarcoplasmic reticulu m that is important for both skeletal and cardiac muscle excitation-co ntraction coupling.