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In vitro interactions between the β_1a subunit of the skeletal muscle DHPR and RyR1

The β_1a subunit of the skeletal muscle dihydropyridine receptor (DHPR) plays two important roles in excitation-contraction (EC) coupling in skeletal muscle. β_1a was originally found to target the α_1S subunit of the DHPR to sarcolemmal tetrads which oppose skeletal ryanodine receptor (RyR1) channels in the sarcoplasmic reticulum. It was later found to also aid in transmission of EC coupling between the α_1S subunit and RyR1, through its C-terminal tail residues. This presumably depends on an interaction between the C-terminal tail of β_1a and RyR1 since the β_1a subunit binds to RyR1 in affinity chromatography experiments, but direct binding of the C-terminal tail to RyR1 has not been reported. Nor have direct functional interactions between the proteins been demonstrated. We show here, using affinity chromatography, that a peptide corresponding to the native sequence of the C-terminus of β_1a, but not a peptide with a scrambled sequence, binds to RyR1 (N=3). In addition the peptide and the full β_1a subunit at 0.1 to 1 nM significantly increase both [³H]ryanodine binding (N=7-18) and single RyR1 channel activity, with maximum 3- to 5-fold activation at ∼10nM (N=7-10 at each of eight concentrations). The increase in RyR1 channel activity with both full length β_1a and the C-terminal peptide was irreversible within the lifetime of the bilayer, indicating high affinity binding. Therefore the C-terminal tail of the β_1a subunit is capable of directly binding to and activating RyR1, suggesting that β_1a may enhance EC coupling by virtue of its ability to activate RyR1.