1. The dihydropyridine receptor (DHPR) II-III loop is an intrinsically unstructured region made up of alpha-helical and beta-turn secondary structure elements with the N and C termini in close spatial proximity. 2. The DHPR II-III loop interacts in vitro with a ryanodine receptor (RyR) 1 SPRY domain through alpha-helical segments located in the A and B regions. Mutations within the A and B regions in the DHPR II-III loop alter the binding affinity to the SPRY2 domain. 3. The A and C peptides derived from DHPR II-III loop show negative cooperativity in binding to the SPRY2 domain. 4. The SPRY2 domain of the RyR1 (1085-1208) forms a beta-sheet sandwich structure flanked by variable loop regions. An acidic loop region of SPRY2 (1107-1121) forms part of a negatively charged cleft that is implicated in the binding of the DHPR II-III loop. 5. The mutant E1108A located in the negatively charged loop of SPRY2 reduces the binding affinity to the DHPR II-III loop.