RESUMEN
Calcium and calmodulin both regulate the skeletal muscle calcium release channel, also known as the ryanodine receptor, RYR1. Ca(2+)-free calmodulin (apocalmodulin) activates and Ca(2+)-calmodulin inhibits the ryanodine receptor. The conversion of calmodulin from an activator to an inhibitor is due to Ca(2+) binding to calmodulin. We have previously shown that the binding sites for apocalmodulin and Ca(2+)-calmodulin on RYR1 are overlapping with the Ca(2+)-calmodulin site located slightly N-terminal to the apocalmodulin binding site. We now show that mutations of the calcium binding sites in either the N-terminal or the C-terminal lobes of calmodulin decrease the affinity of calmodulin for the ryanodine receptor, suggesting that both lobes interact with RYR1. Mutation of the two C-terminal Ca(2+) binding sites of calmodulin destroys calmodulin's ability to inhibit ryanodine receptor activity at high calcium concentrations. The mutated calmodulin, however, can still bind to RYR1 at both nanomolar and micromolar Ca(2+) concentrations. Mutating the two N-terminal calcium binding sites of calmodulin does not significantly alter calmodulin's ability to inhibit ryanodine receptor activity. These data suggest that calcium binding to the two C-terminal calcium binding sites within calmodulin is responsible for the switching of calmodulin from an activator to an inhibitor of the ryanodine receptor.
Asunto(s)
Calcio/metabolismo , Calmodulina/química , Calmodulina/metabolismo , Canal Liberador de Calcio Receptor de Rianodina/metabolismo , Secuencia de Aminoácidos , Animales , Sitios de Unión/genética , Calmodulina/genética , Técnicas In Vitro , Cinética , Modelos Biológicos , Datos de Secuencia Molecular , Mutagénesis Sitio-Dirigida , Fragmentos de Péptidos/química , Fragmentos de Péptidos/genética , Fragmentos de Péptidos/metabolismo , Conejos , Canal Liberador de Calcio Receptor de Rianodina/química , Canal Liberador de Calcio Receptor de Rianodina/genéticaRESUMEN
The skeletal muscle calcium release channel, ryanodine receptor, is activated by calcium-free calmodulin and inhibited by calcium-bound calmodulin. Previous biochemical studies from our laboratory have shown that calcium-free calmodulin and calcium bound calmodulin protect sites at amino acids 3630 and 3637 from trypsin cleavage (Moore, C. P., Rodney, G., Zhang, J. Z., Santacruz-Toloza, L., Strasburg, G., and Hamilton, S. L. (1999) Biochemistry 38, 8532-8537). We now demonstrate that both calcium-free calmodulin and calcium-bound calmodulin bind with nanomolar affinity to a synthetic peptide matching amino acids 3614-3643 of the ryanodine receptor. Deletion of the last nine amino acids (3635-3643) destroys the ability of the peptide to bind calcium-free calmodulin, but not calcium-bound calmodulin. We propose a novel mechanism for calmodulin's interaction with a target protein. Our data suggest that the binding sites for calcium-free calmodulin and calcium-bound calmodulin are overlapping and, when calcium binds to calmodulin, the calmodulin molecule shifts to a more N-terminal location on the ryanodine receptor converting it from an activator to an inhibitor of the channel. This region of the ryanodine receptor has previously been identified as a site of intersubunit contact, suggesting the possibility that calmodulin regulates ryanodine receptor activity by regulating subunit-subunit interactions.
Asunto(s)
Calcio/metabolismo , Calmodulina/metabolismo , Canal Liberador de Calcio Receptor de Rianodina/metabolismo , Secuencia de Aminoácidos , Sitios de Unión , Fluorescencia , Datos de Secuencia Molecular , Unión Proteica , Canal Liberador de Calcio Receptor de Rianodina/química , Triptófano/químicaRESUMEN
Calmodulin, bound to the alpha(1) subunit of the cardiac L-type calcium channel, is required for calcium-dependent inactivation of this channel. Several laboratories have suggested that the site of interaction of calmodulin with the channel is an IQ-like motif in the carboxyl-terminal region of the alpha(1) subunit. Mutations in this IQ motif are linked to L-type Ca(2+) current (I(Ca)) facilitation and inactivation. IQ peptides from L, P/Q, N, and R channels all bind Ca(2+)calmodulin but not Ca(2+)-free calmodulin. Another peptide representing a carboxyl-terminal sequence found only in L-type channels (designated the CB domain) binds Ca(2+)calmodulin and enhances Ca(2+)-dependent I(Ca) facilitation in cardiac myocytes, suggesting the CB domain is functionally important. Calmodulin blocks the binding of an antibody specific for the CB sequence to the skeletal muscle L-type Ca(2+) channel, suggesting that this is a calmodulin binding site on the intact protein. The binding of the IQ and CB peptides to calmodulin appears to be competitive, signifying that the two sequences represent either independent or alternative binding sites for calmodulin rather than both sequences contributing to a single binding site.
Asunto(s)
Canales de Calcio Tipo L/química , Canales de Calcio Tipo L/metabolismo , Calmodulina/metabolismo , Secuencia de Aminoácidos , Animales , Sitios de Unión , Western Blotting , Calcio/metabolismo , Bovinos , Células Cultivadas , Relación Dosis-Respuesta a Droga , Electroforesis en Gel de Poliacrilamida , Electrofisiología , Ensayo de Inmunoadsorción Enzimática , Datos de Secuencia Molecular , Músculo Esquelético/metabolismo , Mutación , Miocardio/citología , Péptidos/química , Péptidos/metabolismo , Unión Proteica , Estructura Terciaria de Proteína , Conejos , Homología de Secuencia de Aminoácido , Espectrometría de FluorescenciaRESUMEN
The skeletal muscle calcium release channel (RYR1) is a Ca(2+)-binding protein that is regulated by another Ca(2+)-binding protein, calmodulin. The functional consequences of calmodulin's interaction with RYR1 are dependent on Ca(2+) concentration. At nanomolar Ca(2+) concentrations, calmodulin is an activator, but at micromolar Ca(2+) concentrations, calmodulin is an inhibitor of RYR1. This raises the question of whether the Ca(2+)-dependent effects of calmodulin on RYR1 function are due to Ca(2+) binding to calmodulin, RYR1, or both. To distinguish the effects of Ca(2+) binding to calmodulin from those of Ca(2+) binding to RYR1, a mutant calmodulin that cannot bind Ca(2+) was used to evaluate the effects of Ca(2+)-free calmodulin on Ca(2+)-bound RYR1. We demonstrate that Ca(2+)-free calmodulin enhances the affinity of RYR1 for Ca(2+) while Ca(2+) binding to calmodulin converts calmodulin from an activator to an inhibitor. Furthermore, Ca(2+) binding to RYR1 enhances its affinity for both Ca(2+)-free and Ca(2+)-bound calmodulin.
Asunto(s)
Proteínas de Unión al Calcio , Calcio/metabolismo , Calmodulina/metabolismo , Proteínas de Drosophila , Canal Liberador de Calcio Receptor de Rianodina/metabolismo , Animales , Calmodulina/genética , Motivos EF Hand , Ácido Glutámico/metabolismo , Técnicas In Vitro , Proteínas de Insectos/metabolismo , Mutagénesis Sitio-Dirigida , Conejos , Canal Liberador de Calcio Receptor de Rianodina/efectos de los fármacos , Canal Liberador de Calcio Receptor de Rianodina/genéticaRESUMEN
We studied the effect of a nitric oxide synthase inhibitor, Nomega-Nitro-L-arginine-methyl-ester (L-NAME), on in vitro diphragmatic function both at rest (control) or after inspiratory resistive loading (IRL). Sprague-Dawley rats were anesthetized, instrumented, and then the following experimental groups: (1) controls; (2) L-NAME (100 mg/kg/body weight intravenously alone); (3) IRL alone; and (4) L-NAME + IRL. The IRL protocol consisted of applying a variable resistor to the inspiratory limb of a two-way valve at 70% of maximal airway pressure until apnea. After the experiment, the animals were sacrificed and diaphragmatic strips were obtained for activity of constitutive nitric oxide synthase (cNOS) and measurements of in vitro contractile properties: tetanic (Po) and twitch tensions (Pt). cNOS activity was significantly decreased in the L-NAME and L-NAME + IRL groups (P < or = 0.05) as compared with control and IRL groups. L-NAME alone did not affect Po or Pt. However, in both IRL groups, with and without was a significant decrease in Po and Pt. This reduction was comparable in both groups. In summary, our data showed that L-NAME resulted in a significant decrease cNOS activity, but in vitro contractility was impaired.