Abstract
The functional relevance of putative Ca(2+) binding motifs previously identified with Ca(2+) overlay binding analysis within the skeletal muscle ryanodine receptor isoform (RyR1) 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 Ca(2+)-imaging techniques, [(3)H]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 [(3)H]ryanodine binding measurements, both Ca(2+) activation and inhibition of the EF1 mutant was significantly altered compared with wtRyR1. No high affinity [(3)H]ryanodine binding was observed in membranes expressing the EF2 mutation, although in single channel measurements, the EF2-disrupted channel could be activated by micromolar Ca(2+) 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 1B5 myotubes except that the EC(50) for caffeine activation for the EF3 construct was significantly increased compared with wtRyR1. However, in [(3)H]ryanodine binding experiments, the Ca(2+)-dependent activation and inactivation of mutated RyRs containing EF3, EF4, or EF5 was unaffected when compared with wtRyR1.
Highlights
Calcium plays a critical role in skeletal muscle contraction
Five putative calcium binding sequences have been proposed in the wtRyR1 primary sequence between amino acid positions 4000 and 4500 (Fig. 1A)
EF hands 1 and 2, identified during the cloning of the lobster RyR1 cDNA [6] are each comprised of a classical helix-loop-helix motif that contains potential calcium-chelating amino acid residues located at the X, Y, Z, ϪX, and ϪZ positions of the calcium binding loop (Fig. 1B)
Summary
Calcium plays a critical role in skeletal muscle contraction. During excitation-contraction coupling, calcium is released from the sarcoplasmic reticulum (SR)1 through activation of the intracellular calcium release channel known as the type 1 ryanodine receptor (RyR1). No high affinity [3H]ryanodine binding was observed in membranes expressing the EF2 mutation, in single channel measurements, the EF2-disrupted channel could be activated by micromolar Ca2؉ concentrations.
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