Cloning, Synthesis and Functional Characterization of a Novel α-Conotoxin Lt1.3

α-Conotoxins (α-CTxs) are small peptides composed of 11 to 20 amino acid residues with two disulfide bridges. Most of them potently and selectively target nicotinic acetylcholine receptor (nAChR) subtypes, and a few were found to inhibit the GABA(B) receptor (GABA(B)R)-coupled N-type calcium channels (Cav2.2). However, in all of α-CTxs targeting both receptors, the disulfide connectivity arrangement “C(1)-C(3), C(2)-C(4)” is present. In this work, a novel α4/7-CTx named Lt1.3 (GCCSHPACSGNNPYFC-NH(2)) was cloned from the venom ducts of Conus litteratus (C. litteratus) in the South China Sea. Lt1.3 was then chemically synthesized and two isomers with disulfide bridges “C(1)-C(3), C(2)-C(4)” and “C(1)-C(4), C(2)-C(3)” were found and functionally characterized. Electrophysiological experiments showed that Lt1.3 containing the common disulfide bridges “C(1)-C(3), C(2)-C(4)” potently and selectively inhibited α3β2 nAChRs and not GABA(B)R-coupled Cav2.2. Surprisingly, but the isomer with the disulfide bridges “C(1)-C(4), C(2)-C(3)” showed exactly the opposite inhibitory activity, inhibiting only GABA(B)R-coupled Cav2.2 and not α3β2 nAChRs. These findings expand the knowledge of the targets and selectivity of α-CTxs and provide a new structural motif to inhibit the GABA(B)R-coupled Cav2.2.