Gabapentin [Neurontin, 1-(aminomethyl)cyclohexaneacetic acid] is a novel anticonvulsant drug with a high binding affinity for the Ca2+-channel subunit α2δ. In this study, the gabapentin-binding properties of wild-type and mutated porcine brain α2δ proteins were investigated. Removal of the disulphide bonds between the α2 and the δ subunits did not result in a significant loss of gabapentin binding, suggesting that the disulphide linkage between the two subunits is not required for binding. Singly expressed α2 protein remained membrane associated. However, α2 alone was unable to bind gabapentin, unless the cells were concurrently transfected with the expression vector for δ, suggesting that both α2 and δ are required for gabapentin binding. Using internal deletion mutagenesis, we mapped two regions [amino acid residues 339-365 (δF) and 875-905 (δJ)] within the α2 subunit that are not required for gabapentin binding. Further, deletion of three other individual regions [amino acid residues 206-222 (δD), 516-537 (δH) and 583-603 (δI)] within the α2 subunit disrupted gabapentin binding, suggesting the structural importance of these regions. Using alanine to replace four to six amino acid residues in each of these regions abolished gabapentin binding. These results demonstrate that region D, between the N-terminal end and the first putative transmembrane domain of α2, and regions H and I, between the putative splicing acceptor sites (Gln511 and Ser601), may play important roles in maintaining the structural integrity for gabapentin binding. Further single amino acid replacement mutagenesis within these regions identified Arg217 as critical for gabapentin binding.
Structural requirement of the calcium-channel subunit α2δ for gabapentin binding