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