Mass spectrometric analysis and mutagenesis predict involvement of multiple cysteines in redox regulation of the skeletal muscle ryanodine receptor ion channel complex

Abstract

The tetrameric skeletal muscle ryanodine receptor ion channel complex (RyR1) contains a large number of free cysteines that are potential targets for redox-active molecules. Here, we report the mass spectrometric analysis of free thiols in RyR1 using the lipophilic, thiol-specific probe monobromobimane (MBB). In the presence of reduced glutathione, MBB labeled 14 cysteines per RyR1 subunit in tryptic peptides in five of five experiments. Forty-six additional MBB-labeled cysteines per RyR1 subunit were detected with lower frequency in tryptic peptides, bringing the total number of MBB-labeled cysteines to 60 per RyR1 subunit. A combination of fluorescence detection and mass spectrometry of RyR1, labeled in the presence of reduced and oxidized glutathione, identified two redox-sensitive cysteines (C1040 and C1303). Regulation of RyR activity by reduced and oxidized glutathione was investigated in skeletal muscle mutant RyR1s in which 18 cysteines were substituted with serine or alanine, using a [(3)H]ryanodine ligand binding assay. Three single-site RyR1 mutants (C1781S, C2436S, and C2606S) and two multisite mutants with five and seven substituted cysteines exhibited a reduced redox response compared with wild-type RyR1. The results suggest that multiple cysteines determine the redox state and activity of RyR1.