Alpha-conotoxins PnIA and [A10L]PnIA stabilize different states of the alpha7-L247T nicotinic acetylcholine receptor

The effects of the native alpha-conotoxin PnIA, its synthetic derivative [A10L]PnIA and alanine scan derivatives of [A10L]PnIA were investigated on chick wild type alpha7 and alpha7-L247T mutant nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes. PnIA and [A10L]PnIA inhibited acetylcholine (ACh)-activated currents at wtalpha7 receptors with IC50 values of 349 and 168 nm, respectively. Rates of onset of inhibition were similar for PnIA and [A10L]PnIA; however, the rate of recovery was slower for [A10L]PnIA, indicating that the increased potency of [A10L]PnIA at alpha7 receptors is conveyed by its slower rate of dissociation from the receptors. All the alanine mutants of [A10L]PnIA inhibited ACh-activated currents at wtalpha7 receptors. Insertion of an alanine residue between position 5 and 13 and at position 15 significantly reduced the ability of [A10L]PnIA to inhibit ACh-evoked currents. PnIA inhibited the non-desensitizing ACh-activated currents at alpha7-L247T receptors with an IC50 194 nm. In contrast, [A10L]PnIA and the alanine mutants potentiated the ACh-activated current alpha7-L247T receptors and in addition [A10L]PnIA acted as an agonist. PnIA stabilized the receptor in a state that is non-conducting in both the wild type and mutant receptors, whereas [A10L]PnIA stabilized a state that is non-conducting in the wild type receptor and conducting in the alpha7-L247T mutant. These data indicate that the change of a single amino acid side-chain, at position 10, is sufficient to change the toxin specificity for receptor states in the alpha7-L247T mutant.