Excitation-contraction coupling in skeletal muscle is thought to depend on a physical interaction between II-III loop of the α₁ subunit of the skeletal dihydropyridine receptor (DHPR) and ryanodine receptor (RyR). A peptide corresponding to II-III loop residues 671-690 of the skeletal DHPR (peptide A) is a high affinity activator of the RyR when it adopts an α-helical structure with critical basic residues aligned along one helical surface (1). Neither the structure of the full length II-III loop, or of sequences longer than 671-690 residues have been determined. Here we describe the structure and function of a 40 amino acid peptide corresponding to residues 671-710 (peptide AB) of the skeletal DHPR α₁ subunit. This peptide contains the A region with a further 20 residues towards the C-terminus of the II-III loop. We predicted that peptide AB would strongly activate the RyR, because (a) it contains the active A sequence of basic residues and (b) it contains a greater proportion of the II-III loop. The structure of the AB peptide was determined and it was found to consist of two helical regions joined by an unstructured linker region. Surprisingly, although the structure of the A region was maintained, the 40 residue peptide was unable to release Ca²⁺ from skeletal SR. Strong activity was restored when four negatively charged residues in the C-terminal part of the peptide were replaced by neutral residues. The charge substitution caused minimal changes in the overall structural profile of the peptide and virtually no changes in the A portion of the peptide. The results suggest that the ability of the A region of the skeletal II-III loop to interact with the RyR could depend on the tertiary conformation of the II-III loop, which is thought to change during EC coupling.