FC0005
Dihydropyridine receptor (DHPR)  -  Ryanodine receptor (Ryr)

Biological function
Excitation–contraction (EC) coupling is the translation of an action potential on the surface membrane of a muscle fibre into Ca²⁺ release from the internal sarcoplasmic reticulum (SR) Ca²⁺ store, to trigger muscle contraction. Two Ca²⁺ channels are important in this signal transmission: the dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR). The DHPR spans the transverse tubular membrane, detects action potential depolarization and communicates the depolarization to the RyR Ca²⁺-release channel in the SR membrane. The mechanism of signalling between the two proteins differs between cardiac and skeletal muscle. In cardiac muscle, a depolarization-induced Ca²⁺ current through the DHPR initiates Ca²⁺-induced Ca²⁺ release from the SR. In skeletal muscle the surface Ca²⁺ current is not necessary for EC coupling. Instead, there is an interaction between the DHPR and RyR, requiring the cytoplasmic loop region between transmembrane repeats II and III of the skeletal DHPR a₁ subunit (Glu666–Leu791). Residues Phe725–Pro742 of the skeletal II–III loop are essential for skeletal-type EC coupling in myocytes.

Structural evidence
The structure of peptide C was examined using NMR and CD. Peptide C has a random-coil structure and therefore adopts different conformations. The peptide may only occasionally achieve the structure necessary to bind to activation or inhibition sites or to activate or inhibit at one site. This means that the concentration of peptide C in its active conformation is in fact lower than that indicated by the total concentration of the peptide.

Biochemical evidence
RyR activation occurs at 100 nm-10 μM DHPR concentration. Peptide C of II-III loop of DHPR (724-760) has moderate affinity for RyR: 0.1-150 μM. and induces two independent stochastic processes. It could either bind to separate activation/inhibition sites, or simply bind to the same site in different conformations, which results in opposite biological effects. Biological data support the first scenario. At least two regions of the II–III loop of the DHPR (peptide A and C) can bind to separate sites on the RyR and that there are functional interactions between these binding sites when the loop regions are bound.

Structure/Mechanism
The most compelling evidence that activation and inhibition by peptide C are independent processes is that in many instances either activation or inhibition was absent, i.e. a significant number of channels showed inhibition without preceding activation, while other channels activated by higher concentrations of peptide C did not show inhibition. The independence of activation or inhibition could have been due to the peptide either (a) binding to separate activation and inhibition sites or (b) binding to one site, sometimes in an activating conformation and at other times in an inhibiting conformation.

Mechanism category
competitive binding

Significance
The effects of peptide C indicate functional interactions between a part of the dihydropyridine receptor and the RyR. These interactions could reflect either dynamic changes that occur during excitation–contraction coupling or interactions between the proteins at rest.

Medical relevance
malignant hyperthermia