Abstract
Neomycin is a potent inhibitor of skeletal muscle sarcoplasmic reticulum (SR) calcium release. To elucidate the mechanism of inhibition, the effects of neomycin on the binding of [3H]ryanodine to the Ca2+ release channel and on its channel activity when reconstituted into planar lipid bilayer were examined. Equilibrium binding of [3H]ryanodine was partially inhibited by neomycin. Inhibition was incomplete at high neomycin concentrations, indicating noncompetitive inhibition rather than direct competitive inhibition. Neomycin and [3H]ryanodine can bind to the channel simultaneously and, if [3H]ryanodine is bound first, the addition of neomycin will slow the dissociation of [3H]ryanodine from the high affinity site. Neomycin also slows the association of [3H]ryanodine with the high affinity binding site. The neomycin binding site, therefore, appears to be distinct from the ryanodine binding site. Dissociation of [3H]ryanodine from trypsin-treated membranes or from a solubilized 14 S complex is also slowed by neomycin. This complex is composed of polypeptides derived from the carboxyl terminus of the Ca2+ release channel after Arg-4475 (Callaway, C., Seryshev, A., Wang, J. P., Slavik, K., Needleman, D. H., Cantu, C., Wu, Y., Jayaraman, T., Marks, A. R., and Hamilton, S. L. (1994) J. Biol. Chem. 269, 15876-15884). The proteolytic 14 S complex isolated with ryanodine bound produces a channel upon reconstitution into planar lipid bilayers, and its activity is inhibited by neomycin. Our data are consistent with a model in which the ryanodine binding sites, the neomycin binding sites, and the channel-forming portion of the Ca2+ release channel are located between Arg-4475 and the carboxyl terminus.
Highlights
Neomycin is a potent inhibitor of skeletal muscle sarcoplasmic reticulum (SR) calcium release
The proteolytic 14 S complex isolated with ryanodine bound produces a channel upon reconstitution into planar lipid bilayers, and its activity is inhibited by neomycin
Our data are consistent with a model in which the ryanodine binding sites, the neomycin binding sites, and the channel-forming portion of the Ca2؉ release channel are located between Arg-4475 and the carboxyl terminus
Summary
To elucidate the mechanism of inhibition, the effects of neomycin on the binding of [3H]ryanodine to the Ca2؉ release channel and on its channel activity when reconstituted into planar lipid bilayer were examined. Dissociation of [3H]ryanodine from trypsin-treated membranes or from a solubilized 14 S complex is slowed by neomycin This complex is composed of polypeptides derived from the carboxyl terminus of the Ca2؉ release channel after Arg4475 Based on the effects of neomycin on the binding of [3H]ryanodine to SR membranes and on 45Ca2ϩ fluxes, Mack et al [10] concluded that neomycin was competitive with ryanodine for high affinity binding sites. We examine the effects of neomycin on [3H]ryanodine binding and on the behavior of the Ca2ϩ release channel incorporated into planar lipid bilayers to demonstrate that ryanodine and neomycin bind noncompetitively to the channel
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