Fluorescence Resonance Energy Transfer-based Structural Analysis of the Dihydropyridine Receptor α1S Subunit Reveals Conformational Differences Induced by Binding of the β1a Subunit

Abstract

The skeletal muscle dihydropyridine receptor ␣ 1S subunit plays a key role in skeletal muscle excitation-contraction coupling by sensing membrane voltage changes and then triggering intracellular calcium release. The cytoplasmic loops connecting four homologous ␣ 1S structural domains have diverse functions, but their structural arrangement is poorly understood. Here, we used a novel FRET-based method to characterize the relative proximity of these intracellular loops in ␣ 1S subunits expressed in intact cells. In dysgenic myotubes, energy transfer was observed from an N-terminal-fused YFP to a FRET acceptor, ReAsH (resorufin arsenical hairpin binder), targeted to each ␣ 1S intracellular loop, with the highest FRET efficiencies measured to the ␣ 1S II-III loop and C-terminal tail. However, in HEK-293T cells, FRET efficiencies from the ␣ 1S N terminus to the II-III and III-IV loops and the C-terminal tail were significantly lower, thus suggesting that these loop structures are influenced by the cellular microenvironment. The addition of the ␤ 1a dihydropyridine receptor subunit enhanced FRET to the II-III loop, thus indicating that ␤ 1a binding directly affects II-III loop conformation. This specific structural change required the C-terminal 36 amino acids of ␤ 1a , which are essential to support EC coupling. Direct FRET measurements between ␣ 1S and ␤ 1a confirmed that both wild type and truncated ␤ 1a bind similarly to ␣ 1S . These results provide new insights into the role of muscle-specific proteins on the structural arrangement of ␣ 1S intracellular loops and point to a new conformational effect of the ␤ 1a subunit in supporting skeletal muscle excitation-contraction coupling.In skeletal muscle excitation contraction (EC) 3 coupling, the dihydropyridine receptor (DHPR), an L-type voltage-gated Ca 2ϩ channel, senses membrane depolarization and then initiates intracellular Ca 2ϩ release by activating the type 1 ryanodine receptor (RyR1) embedded in the sarcoplasmic reticulum membrane. Of the five skeletal DHPR subunits, ␣ 1S and ␤ 1a are absolutely required for skeletal type EC coupling (1-8). The 170-kDa ␣ 1S subunit contains both the voltage sensor and Ca 2ϩ conduction pore and is composed of four homologous domains, each containing six transmembrane segments (9, 10). These domains are connected by intracellular loops with well defined functions. For example the I-II loop has an ␣ 1S subunit interaction domain binding site for the ␤ 1a DHPR subunit (11, 12), whereas the II-III loop contains an essential determinant (amino acids 720 -765) required to activate RyR1 during skeletal-type EC coupling (13,14). Although the III-IV loop does not appear to have a direct role in RyR1 activation, a malignant hyperthermia mutation located in this loop (R1086H) has been reported to alter DHPR gating properties and EC coupling (15). Finally, the C-terminal tail has been implicated as binding Ca 2ϩ /calmodulin (16) as well as mediating proper DHPR targeting to triad junctions (17). Although a recent high resolution cryo-EM...

Core skeletal muscle ryanodine receptor calcium release complex

“…However, it is worth considering that the SH3 binding site may become available in the intact muscle fibre allowing II‐III loop interactions to occur. Interestingly, fluorescence resonance energy transfer (FRET) data shows that β 1a binding to α 1S alters II‐III loop conformation . Additionally, a mutation in the SH3 domain of β 1a linked to malignant hyperthermia, indicates that subtle changes to this region impact on physiological function.…”