1H, 13C and 15N assignments for the II-III loop region of the skeletal dyhydropyridine receptor

Molecular interactions of STAC proteins with skeletal muscle dihydropyridine receptor and excitation‐contraction coupling

Excitation‐contraction coupling (ECC) is the physiological process in which an electrical signal originating from the central nervous system is converted into muscle contraction. In skeletal muscle tissue, the key step in the molecular mechanism of ECC initiated by the muscle action potential is the cooperation between two Ca²⁺ channels, dihydropyridine receptor (DHPR; voltage‐dependent L‐type calcium channel) and ryanodine receptor 1 (RyR1). These two channels were originally postulated to communicate with each other via direct mechanical interactions; however, the molecular details of this cooperation have remained ambiguous. Recently, it has been proposed that one or more supporting proteins are in fact required for communication of DHPR with RyR1 during the ECC process. One such protein that is increasingly believed to play a role in this interaction is the SH3 and cysteine‐rich domain‐containing protein 3 (STAC3), which has been proposed to bind a cytosolic portion of the DHPR α_1S subunit known as the II–III loop. In this work, we present direct evidence for an interaction between a small peptide sequence of the II–III loop and several residues within the SH3 domains of STAC3 as well as the neuronal isoform STAC2. Differences in this interaction between STAC3 and STAC2 suggest that STAC3 possesses distinct biophysical features that are potentially important for its physiological interactions with the II–III loop. Therefore, this work demonstrates an isoform‐specific interaction between STAC3 and the II–III loop of DHPR and provides novel insights into a putative molecular mechanism behind this association in the skeletal muscle ECC process.

A dihydropyridine receptor alpha1s loop region critical for skeletal muscle contraction is intrinsically unstructured and binds to a SPRY domain of the type 1 ryanodine receptor

The II-III loop of the dihydropyridine receptor (DHPR) alpha(1s) subunit is a modulator of the ryanodine receptor (RyR1) Ca(2+) release channel in vitro and is essential for skeletal muscle contraction in vivo. Despite its importance, the structure of this loop has not been reported. We have investigated its structure using a suite of NMR techniques which revealed that the DHPR II-III loop is an intrinsically unstructured protein (IUP) and as such belongs to a burgeoning structural class of functionally important proteins. The loop does not possess a stable tertiary fold: it is highly flexible, with a strong N-terminal helix followed by nascent helical/turn elements and unstructured segments. Its residual structure is loosely globular with the N and C termini in close proximity. The unstructured nature of the II-III loop may allow it to easily modify its interaction with RyR1 following a surface action potential and thus initiate rapid Ca(2+) release and contraction. The in vitro binding partner for the II-III was investigated. The II-III loop interacts with the second of three structurally distinct SPRY domains in RyR1, whose function is unknown. This interaction occurs through two preformed N-terminal alpha-helical regions and a C-terminal hydrophobic element. The A peptide corresponding to the helical N-terminal region is a common probe of RyR function and binds to the same SPRY domain as the full II-III loop. Thus the second SPRY domain is an in vitro binding site for the II-III loop. The possible in vivo role of this region is discussed.

Excitation-contraction coupling from the 1950s into the new millennium

1. Excitation-contraction coupling is broadly defined as the process linking the action potential to contraction in striated muscle or, more narrowly, as the process coupling surface membrane depolarization to Ca(2+) release from the sarcoplasmic reticulum. 2. We now know that excitation-contraction coupling depends on a macromolecular protein complex or 'calcium release unit'. The complex extends the extracellular space within the transverse tubule invaginations of the surface membrane, across the transverse tubule membrane into the cytoplasm and then across the sarcoplasmic reticulum membrane and into the lumen of the sarcoplasmic reticulum. 3. The central element of the macromolecular complex is the ryanodine receptor calcium release channel in the sarcoplasmic reticulum membrane. The ryanodine receptor has recruited a surface membrane L-type calcium channel as a 'voltage sensor' to detect the action potential and the calcium-binding protein calsequestrin to detect in the environment within the sarcoplasmic reticulum. Consequently, the calcium release channel is able to respond to surface depolarization in a manner that depends on the Ca(2+) load within the calcium store. 4. The molecular components of the 'calcium release unit' are the same in skeletal and cardiac muscle. However, the mechanism of excitation-contraction coupling is different. The signal from the voltage sensor to ryanodine receptor is chemical in the heart, depending on an influx of external Ca(2+) through the surface calcium channel. In contrast, conformational coupling links the voltage sensor and the ryanodine receptor in skeletal muscle. 5. Our current understanding of this amazingly efficient molecular signal transduction machine has evolved over the past 50 years. None of the proteins had been identified in the 1950s; indeed, there was debate about whether the molecules involved were, in fact, protein. Nevertheless, a multitude of questions about the molecular interactions and structures of the proteins and their interaction sites remain to be answered and provide a challenge for the next 50 years.

STRUCTURAL AND FUNCTIONAL CHARACTERIZATION OF INTERACTIONS BETWEEN THE DIHYDROPYRIDINE RECEPTOR II?III LOOP AND THE RYANODINE RECEPTOR

1. Excitation-contraction coupling in skeletal muscle is dependent on a physical interaction between the dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR). 2. A number of peptides derived from the II-III loop region of the DHPR have been shown to be functionally active in stimulating the release of calcium via RyR channels. Their function has been found to correlate with the presence of a basic helical region located at the N-terminus of the II-III loop. 3. The entire recombinant skeletal DHPR II-III loop is an efficient activator of RyR1 and RyR2. 4. The skeletal DHPR II-III loop is comprised of a series of a-helices, but its tertiary structure has been determined to be unstructured and flexible. 5. Fluorescence quenching experiments have been used to identify and measure the binding affinity of the II-III loop with fragments of the RyR.