The functional relevance of putative Ca2+ binding motifs previously identified with Ca2+ overlay binding analysis within the skeletal muscle ryanodine receptor isoform (MyR1) was examined using mutational analysis. EF hands between amino acid positions 4081 and 4092 (EF1) and 4116 and 4127 (EF2) were scrambled singly or in combination within the full-length rabbit RyR1 cDNA. These cDNAs were expressed in 1B5 RyR-deficient myotubes and channel function assessed using Ca2+-imaging techniques, [ 3H]ryanodine binding measurements, and single channel experiments. In intact myotubes, these mutations did not affect functional responses to either depolarization or RyR agonists (caffeine, 4-chloro-m-cresol) compared with wtRyR1. However, in [3H]ryanodine binding measurements, both Ca 2+ activation and inhibition of the EF1 mutant was significantly altered compared with wtRyR1. No high affinity [3H]ryanodine binding was observed in membranes expressing the EF2 mutation, although in single channel measurements, the EF2-disrupted channel could be activated by micromolar Ca2+ concentrations. In addition, micromolar levels of ryanodine placed these channels into the classical half-conductance state, thus indicating that occupancy of high affinity ryanodine binding sites is not required for ryanodine-induced subconductance states in RyR1. Disruption of three additional putative RyR1 calcium binding motifs located between amino acid positions 4254 and 4265 (EF3), 4407 and 4418 (EF4), or 4490 and 4502 (EF5) either singly or in combination (EF3-5) did not affect functional responses in IB5 myotubes except that the EC50 for caffeine activation for the EF3 construct was significantly increased compared with WtRyR1. However, in [3H] ryanodine binding experiments, the Ca2+-dependent activation and inactivation of mutated RyRs containing EF3, EF4, or EF5 was unaffected when compared with wtRyR1.
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