The action of carboxyl modifying reagents on the ryanodine receptor/Ca2+ release channel of skeletal muscle sarcoplasmic reticulum

Mutation of divergent region 1 alters caffeine and Ca(2+) sensitivity of the skeletal muscle Ca(2+) release channel (ryanodine receptor)

Replacement of amino acids 4187-4628 in the skeletal muscle Ca(2+) release channel (skeletal ryanodine receptor (RyR1)), including nearly all of divergent region 1 (amino acids 4254-4631), with the corresponding cardiac ryanodine receptor (RyR2) sequence leads to increased sensitivity of channel activation by caffeine and Ca(2+) and to decreased sensitivity of channel inactivation by elevated Ca(2+) (Du, G. G., and MacLennan, D. H. (1999) J. Biol. Chem. 274, 26120-26126). In further investigations, this region was subdivided by the construction of new chimeras, and alterations in channel function were detected by measurement of the caffeine dependence of in vivo Ca(2+) release and the Ca(2+) dependence of [(3)H]ryanodine binding. Chimera RF10a (amino acids 4187-4381) had a lower EC(50) value for activation by caffeine, and RF10c (4557-4628) had a higher EC(50) value, whereas the EC(50) value for chimera RF10b (4382-4556) was unchanged. Chimeras RF10b and RF10c were more sensitive to activation by Ca(2+), whereas RF10a was less sensitive to inactivation by Ca(2+), implicating RF10b and RF10c in Ca(2+) activation and RF10a in Ca(2+) inactivation. Deletion of much of divergent region 1 sequence to create mutant Delta4274-4535 led to higher caffeine and Ca(2+) sensitivity of channel activation and to lower Ca(2+) sensitivity for inactivation. Thus, deletion results demonstrate that caffeine, Ca(2+), and ryanodine binding sites are not located in amino acids 4274-4535. Nevertheless, the properties of the deletion and chimeric mutants demonstrate that amino acids 4274-4535 and three shorter sequences in this region (F10a, amino acids 4187-4381; F10b, 4382-4556; and F10c, 4557-4628) in RyR1 modulate Ca(2+) and caffeine sensitivity of the Ca(2+) release channel.

Ca(2+) inactivation sites are located in the COOH-terminal quarter of recombinant rabbit skeletal muscle Ca(2+) release channels (ryanodine receptors)

Ca(2+) activation of skeletal (RyR1) and cardiac (RyR2) muscle Ca(2+) release channels (ryanodine receptors) occurs with EC(50) values of about 1 microM. Ca(2+) inactivation occurs with an IC(50) value of about 3.7 mM for RyR1, but RyR2 shows little inactivation, even at >100 mM Ca(2+). In an attempt to localize the low affinity Ca(2+) binding sites responsible for Ca(2+) inactivation in RyR1, chimeric RyR1/RyR2 molecules were constructed. Because [(3)H]ryanodine binds only to open channels, and because channel opening and closing are Ca(2+)-dependent, the Ca(2+) dependence of [(3)H]ryanodine binding was used as an indirect measurement of Ca(2+) release channel opening and closing. IC(50) values for [(3)H]ryanodine binding suggested that Ca(2+) affinity for the low affinity Ca(2+) inactivation sites was unchanged in a chimera in which a glutamate-rich sequence (amino acids 1743-1964) in RyR1 was replaced with the corresponding, less acidic sequence from RyR2. Ca(2+) affinity (IC(50)) for low affinity Ca(2+) inactivation sites was intermediate in RyR1/RyR2 chimeras containing RyR2 amino acids 3726-4186 (RF9), 4187-4628 (RF10), or 4629-5037 (RF11), was closer to RyR2 values in RyR1 chimeras with longer RyR2 replacements (RF9/10 or RF10/11), and was indistinguishable from RyR2 in RyR1 containing all three RyR2 replacements (RF9/10/11). These data suggest that multiple low affinity Ca(2+) binding sites or multiple components of a low affinity Ca(2+) binding site are located between amino acids 3726 and 5037 and that their effects on Ca(2+) inactivation of the release channel are cooperative. Measurement of Ca(2+) activation of [(3)H]ryanodine binding showed that chimeras RF10, RF9/10, and RF9/10/11 were more sensitive to Ca(2+) than was either RyR1 or RyR2. Measurement of caffeine activation of Ca(2+) release in vivo showed that chimeras RF9, RF10, RF9/10, RF10/11, and RF9/10/11 were more sensitive to caffeine than wild-type RyR1. These results suggest that Ca(2+) and caffeine activation sites also involve COOH-terminal sequences in RyR1 and RyR2.

Modification of ryanodine receptor/Ca2+ release channel with dinitrofluorobenzene

Modification of the ryanodine receptor (RyR)/Ca(2+) release channel with 2,4-dinitrofluorobenzene (DNFB) indicated that two classes of amino group interact with the reagent, as can be distinguished on the basis of their reactivity/accessibility and the effects on ryanodine binding and single channel activities. One group interacted very rapidly (t(1/2)<30 s) at 25 degrees C with low concentrations of DNFB [C(50) (concentration of DNFB required for 50% inhibition or stimulation of ryanodine binding)=5 microM], and at pH values of 6.2 and higher. This interaction resulted in the marked stimulation of ryanodine binding and the complete inhibition of a single Ca(2+) release channel incorporated into planar lipid bilayer. The second group is accessible at higher temperatures (37 degrees C); at pH values higher than 7.4 it reacted slowly (t(1/2)=20 min) with high concentrations of DNFB (C(50)=70 microM). This interaction led to the inhibition of ryanodine binding and single channel activity. Modification of RyR with DNFB under the stimulatory conditions resulted in 3.6-fold and 6-fold increases in ryanodine-binding and Ca(2+)-binding affinities respectively. Modification with DNFB under the inhibitory conditions resulted in a decrease in the total ryanodine-binding sites. The exposure of the RyR single channel to DNFB under both inhibitory and stimulatory conditions led to the complete closure of the channel. However, when modified under the stimulatory conditions, but not under the inhibitory ones, the DNFB-modified closed channel could be re-activated by sub-micromolar concentrations of ryanodine, in the presence of nanomolar concentrations of Ca(2+). The DNFB-modified ryanodine-activated RyR channel showed fast transitions between open, closed and several sub-conductance states, and was completely closed by Ruthenium Red. ATP re-activated the DNFB-modified closed channel or, if present during modification, prevented the inhibition of RyR channel activity by DNFB. Neither the stimulation nor the inhibition of ryanodine binding by modification with DNFB was affected by the presence of ATP. By using the photoreactive ATP analogue 3'-O-(4-benzoyl)benzoyl-[alpha-(32)P]ATP we found that DNFB modification had no effect on the ATP-binding site of RyR. The results are discussed with regard to the involvement of amino group residues in channel gating, ryanodine association/dissociation and occlusion, and the relationship between the open/closed state of the RyR and its capacity to bind ryanodine.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

Involvement of lysine residues in the gating of the ryanodine receptor/Ca2+-release channel of skeletal muscle sarcoplasmic reticulum

In this study, the modification of skeletal muscle ryanodine receptor (RyR)/Ca2+-release channel with 7-chloro-4-nitrobenzo-2-oxa-1,3,-diazole (Nbd-Cl) demonstrates that lysyl residues are involved in the channel gating. Nbd-Cl was found to have a dual effect: stimulation and inhibition of ryanodine binding and single channel activities. Nbd-Cl, in a time-dependent manner, first stimulated and subsequently inhibited ryanodine binding to both membrane-bound and purified RyR. Incubation of sacroplasmic reticulum membranes with Nbd-Cl for 5-20 s resulted in enhanced ryanodine-binding activity by 2-4-fold due, to an increased binding affinity by about tenfold, with no effect on the total binding sites (Bmax). However, under prolonged incubation (5-20 min), Nbd-Cl strongly inhibited ryanodine binding by decreasing the Bmax with no effect on the binding affinity. Similar effects of stimulation and inhibition by Nbd-Cl were obtained with single channel activity of RyR reconstituted into planar lipid bilayer. Nbd-Cl initially (within a few seconds) activated the channel to a highly open state, then (within a few minutes) inactivated it to the completely closed state. Nbd-Cl-modified protein, as assayed by ryanodine binding or single channel activities, was stable against thiolysis by dithiothreitol, suggesting Nbd-Cl modification of lysyl residues. Evidence from absorption and fluorescence excitation and emission spectra also demonstrated that lysyl residues in RyR were modified by Nbd-Cl. Spectrophotometric data were used to estimate a ratio of up to 1 mol Nbd bound/mol RyR (tetramer) and up to 4 mol Nbd bound per mol RyR (tetramer) for Nbd-Cl stimulated and inhibited RyR activities, respectively. The results clearly indicate the involvement of two classes of lysyl residues in RyR activity. Modification by Nbd-Cl of the fast-reacting group led to stimulation of ryanodine binding and single channel activities, while modification of the slow-reacting group resulted in inhibition of these activities. Thus, the involvement of lysine residues in the gating of the RyR channel is proposed.