Activation of ryanodine receptors by imperatoxin A and a peptide segment of the II-III loop of the dihydropyridine receptor

Excitation-contraction coupling in skeletal muscle is believed to be trigge red by direct protein-protein interactions between the sarcolemmal dihydrop yridine-sensitive Ca2+ channel and the Ca2+ release channel/ryanodine recep tor (RyR) of sarcoplasmic reticulum. A 138-amino acid cytoplasmic loop betw een repeats II and III of the (alpha(1), subunit of the skeletal dihydropyr idine receptor (the II-III loop) interacts with a region of the RyR to elic it Ca2+ release. In addition, small segments (10-20 amino acid residues) of the II-III loop retain the capacity to activate Ca2+ release. Imperatoxin A, a 33-amino acid peptide from the scorpion Pandinus imperator, binds dire ctly to the RyR and displays structural and functional homology with an act ivating segment of the II-III loop (Glu(666)-Leu(690)), Mutations in a stru ctural motif composed of a cluster of basic amino acids followed by Ser or Thr dramatically reduce or completely abolish the capacity of the peptides to activate RyRs, Thus, the Imperatoxin A-RyR interaction mimics critical m olecular characteristics of the II-III loop-RyR interaction and may be a us eful tool to elucidate the molecular mechanism that couples membrane depola rization to sarcoplasmic reticulum Ca2+ release in vivo.