A role for the β12 loop in the gating of 5-HT3 receptors

David C. Reeves, Michaela Jansen, Moez Bali, Thomas Lemster, Myles H. Akabas

Research output: Contribution to journalArticlepeer-review

59 Scopus citations


Based on the Torpedo acetylcholine receptor structure, Unwin and colleagues (Miyazawa et al., 2003; Unwin, 2005) hypothesized that the transduction of agonist binding to channel gate opening involves a "pin-into-socket" interaction between αV46 at the tip of the extra-cellular β12 loop and the transmembrane M2 segment and M2-M3 loop. We mutated to cysteine the aligned positions in the 5-HT 3A and 5-HT3B subunit β1- β2 loops K81 and Q70, respectively. The maximal 5-HT-activated currents in receptors containing 5-HT3A/K81C or 5-HT 3B/Q70C were markedly reduced compared with wild type. Desensitization of wild-type currents involved fast and slow components. Mutant currents desensitized with only the fast time constant. Reaction with several methanethiosulfonate reagents potentiated currents to wild-type levels, but reaction with other more rigid thiol-reactive reagents caused inhibition. Single-channel conductances of wild type, K81C, and K81C after modification were similar. We tested the proximity of K81C to the M2-M3 loop by mutating M2-M3 loop residues to cysteine in the K81C background. Bisulfide bonds formed in 5-HT3A/K81C/A304C and 5-HT3A/K81C/I305C when coexpressed with 5-HT3B. We conclude that in the resting state, K81 is not in a hydrophobic pocket as suggested by the pin-into-socket hypothesis. K81 interacts with the extracellular end of M2 and plays a critical role in channel opening and in the return from fast desensitization. We suggest that during channel activation, β12 loop movement moves M2 and the M2-M3 loop so that the M2 segments rotate/translate away from the channel axis, thereby opening the lumen. Recovery from fast desensitization requires the interaction between K81 and the extracellular end of M2.

Original languageEnglish (US)
Pages (from-to)9358-9366
Number of pages9
JournalJournal of Neuroscience
Issue number41
StatePublished - Oct 12 2005


  • Acetylcholine
  • Cross-linking
  • GABA
  • Ion channel
  • SCAM
  • Serotonin

ASJC Scopus subject areas

  • Neuroscience(all)


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