title: Modulation of cardiac ryanodine receptors by sorcin.
abstract: Sorcin is a widely expressed, 22-kDa Ca2+-binding protein initially identified in multidrug-resistant cells. In the heart, sorcin localizes to the dyadic junctions of transverse tubules and sarcoplasmic reticulum and coimmunoprecipitates with the Ca2+ release channel/ryanodine receptor (RyR) (Meyers, M. B., Pickel, V. M., Sheu, S.-S., Sharma, V. K., Scotto, K. W., and Fishman, G. I. (1995) J. Biol. Chem. 270, 26411-26418). We have investigated a possible functional interaction between sorcin and cardiac RyR using purified recombinant sorcin in [3H]ryanodine binding experiments and single channel recordings of RyR. The open probability of single RyR was decreased significantly by the addition of sorcin to the cytoplasmic side of the channel (IC50 approximately 480 nM). In addition, sorcin completely inhibited [3H]ryanodine binding with an IC50 approximately 700 nM. Inhibition occurred over a wide range of [Ca2+], and sorcin-modulated RyR remained Ca2+-dependent. Furthermore, caffeine-activated RyRs were also inhibited by sorcin at low [Ca2+] (pCa 7), suggesting that Ca2+ is not an obligatory factor for sorcin inhibition of RyR. Comparisons of these inhibitory effects with those of calmodulin and calpain, proteins structurally related to sorcin, suggested that the interaction of sorcin with cardiac RyR was distinct from and independent of either of these modulatory proteins. Phosphorylation of sorcin with the catalytic subunit of protein kinase A significantly decreased the ability of sorcin to modulate RyR. These results suggest that sorcin may modulate RyR function in a normal cell environment and that the level of modulation is in turn influenced by signaling pathways that increase protein kinase A activity.
authors.affiliation: Department of Physiology, University of Wisconsin Medical School, Madison, Wisconsin 53706, USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: Lokuta
authors.forename: M B
authors.initials: MB
authors.lastname: Meyers
authors.forename: P R
authors.initials: PR
authors.lastname: Sander
authors.forename: G I
authors.initials: GI
authors.lastname: Fishman
authors.forename: H H
authors.initials: HH
authors.lastname: Valdivia
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002135:Calcium-Binding Proteins / Q000378:metabolism / Q000494:pharmacology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D006321:Heart / Q000502:physiology*; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008861:Microsomes / Q000187:drug effects / Q000502:physiology*; D009124:Muscle Proteins / Q000187:drug effects / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011336:Probability; D011994:Recombinant Proteins / Q000378:metabolism / Q000494:pharmacology; D012433:Ryanodine / Q000378:metabolism; D019837:Ryanodine Receptor Calcium Release Channel; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D013552:Swine
chemical_list: D015220:Calcium Channels; D002135:Calcium-Binding Proteins; D009124:Muscle Proteins; D010750:Phosphoproteins; D011994:Recombinant Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D002110:Caffeine; D017868:Cyclic AMP-Dependent Protein Kinases; D002118:Calcium