D alpha3, a new functional alpha subunit of nicotinic acetylcholine receptors from Drosophila

Nicotinic acetylcholine (ACh) receptors (nAChRs) are important excitatory neurotransmitter receptors in the insect CNS. We have isolated and characterized the gene and the cDNA of a new nAChR subunit from Drosophila. The predicted mature nAChR protein consists of 773 amino acid residues and has the structural features of an ACh-binding alpha subunit. It was therefore named D alpha3, for Drosophila alpha-subunit 3. The d alpha3 gene maps to the X chromosome at position 7E. The properties of the D alpha3 protein were assessed by expression in Xenopus oocytes. D alpha3 did not form functional receptors on its own or in combination with any Drosophila beta-type nAChR subunit. Nondesensitizing ACh-evoked inward currents were observed when D alpha3 was coexpressed with the chick beta2 subunit. Half-maximal responses were at approximately 0.15 microM ACh with a Hill coefficient of approximately 1.5. The snake venom component alpha-bungarotoxin (100 nM) efficiently but reversibly blocked D alpha3/beta2 receptors, suggesting that D alpha3 may be a component of one of the previously described two classes of toxin binding sites in the Drosophila CNS.