Three amino acid residues determine selective binding of FK506-binding protein 12.6 to the cardiac ryanodine receptor

FK506-binding protein (FKBP12) has been found to be associated with the ske letal muscle ryanodine receptor (RyR1) (calcium release channel), whereas F KBP12.6, a novel isoform of FKBP, is selectively associated with the cardia c ryanodine receptor (RyR2), For both RyRs, the stoichiometry is 4 FKBP/RyR . Although FKBP12.6 differs from FKBP12 by only 18 of 108 amino acids, FKBP 12.6 selectively binds to RyR2 and exchanges with bound FKBP12.6 of RyR2, w hereas both FKBP isoforms bind to RyR1 and exchange with bound FKBP12 of Ry R1. To assess the amino acid residues of FKBP12.6 that are critical for sel ective binding to RyR2, the residues of FKBP12.6 that differ with FKBP12 we re mutated to the respective residues of FKBP12. RyR2 of cardiac sarcoplasm ic reticulum, prelabeled by exchange with [S-35]FKBP12.6, was used as assay system for binding/ exchange with the mutants. The triple mutant (Q31E/ N3 2D/F59W) of FKBP12.6 was found to lack selective binding to the cardiac RyR 2, comparable with that of FKBP12.0. In complementary studies, mutations of FKBP12 to the three critical amino acids of FKBP12.6, conferred selective binding to RyR2, Each of the FKBP12.6 and FKBP12 mutants retained binding t o the skeletal muscle RyR1, We conclude that three amino acid residues (Gln (31), Asn(32), and Phe(59)) of human FKBP12.6 account for the selective bin ding to cardiac RyR2.