Triadin isoforms, splice variants of one gene, maintain healthy Ca2+ homeostasis in skeletal muscle by subserving several functions including an influence on Ca2+ release through the ligand-gated ryanodine receptor (RyR1) ion channels. The predominant triadin isoform in skeletal muscle, Trisk 95, activates RyR1 in vitro via binding to previously unidentified amino acids between residues 200 and 232. Here, we identify three amino acids that influence Trisk 95 binding to RyR1 and ion channel activation, using peptides encompassing residues 200–232. Selective alanine substitutions show that K218, K220, and K224 together facilitate normal Trisk 95 binding to RyR1 and channel activation. Neither RyR1 binding nor activation are altered by alanine substitution of K220 alone or of K218 and K224. Therefore K218, K220, and K224 contribute to a robust binding and activation site that is disrupted only when the charge on all three residues is neutralized. We suggest that charged pair interactions between acidic RyR1 residues D4878, D4907, and E4908 and Trisk 95 residues K218, K220, and K224 facilitate Trisk 95 binding to RyR1 and channel activation. Since K218, K220, and K224 are also required for CSQ binding to RyRs (Kobayashi et al. 17, J Biol Chem 275, 17639–17646), the results suggest that Trisk 95 may not simultaneously bind to RyR1 and CSQ, contrary to the widely held belief that triadin monomers form a quaternary complex with junctin, CSQ and RyR1. Therefore, the in vivo role of triadin monomers in modulating RyR1 activity is likely unrelated to CSQ.
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We thank S Pace and J Stivala for the preparation of SR vesicles and purification of RyR1. This work was supported by the Australian Research Council (DP1094219 to A.F.D. and N. A. B.), a NHMRC Career Development Award (APP1003985 to NAB) and an Australian Postgraduate Award PhD scholarship to E.W.).
Compliance with ethical standards
EW: designed research, performed research, undertook analysis, contributed to writing of manuscript.
AD: designed research, undertook analysis, contributed to writing of manuscript.
NB: designed research, undertook analysis, contributed to writing of manuscript.
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