Common determinants of single channel conductance within the large cytoplasmic loop of 5-hydroxytryptamine type 3 and alpha4beta2 nicotinic acetylcholine receptors.

Homomeric 5-hydroxytryptamine type 3A receptors (5-HT3ARs) have a single channel conductance (?) below the resolution of single channel recording (966 ± 75 fS, estimated by variance analysis). By contrast, heteromeric 5-HT3A/B and nicotinic acetylcholine receptors (nAChRs) have picosiemen range ? values. In this study, single channel recordings revealed that replacement of cytoplasmic membrane-associated (MA) helix arginine 432 (-4?), 436 (0?), and 440 (4?) residues by 5-HT3B (-4?Gln, 0?Asp, and 4?Ala) residues increases ? to 36.5 ± 1.0 pS. The 0? residue makes the most substantial contribution to ? of the 5-HT3AR. Replacement of 0?Arg by aspartate, glutamate (?7 nAChR subunit MA 0?), or glutamine (?2 subunit MA 0?) increases ? to the resolvable range (>6 pS). By contrast, replacement of 0?Arg by phenylalanine (?4 subunit MA 0?) reduced ? to 416 ± 107 fS. In reciprocal experiments with ?4?2 nAChRs (? = 31.3 ± 0.8 pS), replacement of MA 0? residues by arginine in ?4?2(Q443R) and ?4(F588R)?2 reduced ? slightly. By contrast, the ? of double mutant ?4(F588R)?2(Q443R) was halved. The MA -4? and 4? residues also influenced ? of 5-HT3ARs. Replacement of nAChR ?4 or ?2 MA 4? residues by arginine made current density negligible. By contrast, replacement of both -4? residues by arginine produced functional nAChRs with substantially reduced ? (11.4 ± 0.5 pS). Homology models of the 5-HT3A and ?4?2 nAChRs against Torpedo nAChR revealed MA -4?, 0?, and 4? residues within five intracellular portals. This locus may be a common determinant of ion conduction throughout the Cys loop receptor family.