Excitation-Contraction Coupling Is Unaffected by Drastic Alteration of the Sequence Surrounding Residues L720-L764 of the α 1S II-III Loop

The II-III loop of the skeletal muscle dihydropyridine receptor (DHPR) α 1S subunit is responsible for bidirectional-signaling interactions with the ryanodine receptor (RyR1): transmitting an orthograde, excitation-contraction (EC) coupling signal to RyR1 and receiving a retrograde, current-enhancing signal from RyR1. Previously, several reports argued for the importance of two distinct regions of the skeletal II-III loop (residues R681-L690 and residues L720-Q765, respectively), claiming for each a key function in DHPR-RyR1 communication. To address whether residues 720-765 of the II-III loop are sufficient to enable skeletal-type (Ca 2+ entry-independent) EC coupling and retrograde interaction with RyR1, we constructed a green fluorescent protein (GFP)-tagged chimera (GFP-SkLM) having rabbit skeletal (Sk) DHPR sequence except for a II-III loop (L) from the DHPR of the house fly, Musca domestica (M). The Musca II-III loop (75% dissimilarity to α 1S) has no similarity to α 1S in the regions R681-L690 and L720-Q765. GFP-SkLM expressed in dysgenic myotubes (which lack endogenous α 1S subunits) was unable to restore EC coupling and displayed strongly reduced Ca 2+ current densities despite normal surface expression levels and correct triad targeting (colocalization with RyR1). Introducing rabbit α 1S residues L720-L764 into the Musca II-III loop of GFP-SkLM (substitution for Musca DHPR residues E724-T755) completely restored bidirectional coupling, indicating its dependence on α 1S loop residues 720-764 but its independence from other regions of the loop. Thus, 45 α 1S-residues embedded in a very dissimilar background are sufficient to restore bidirectional coupling, indicating that these residues may be a site of a protein-protein interaction required for bidirectional coupling.