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Cellular/Molecular A Role for the 1 2 Loop in the Gating of 5-HT3 Receptors
 

Summary: Cellular/Molecular
A Role for the 1 2 Loop in the Gating of 5-HT3 Receptors
David C. Reeves,1 Michaela Jansen,1 Moez Bali,1 Thomas Lemster,2 and Myles H. Akabas1
1Department of Physiology and Biophysics and Department of Neuroscience, Albert Einstein College of Medicine of Yeshiva University, Bronx, New York
10461, and 2Department of Medicinal Chemistry, Institute of Pharmacy, Johannes Gutenberg University, D-55099 Mainz, Germany
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 V46 at the tip of the extra-
cellular 1 2 loop and the transmembrane M2 segment and M2M3 loop. We mutated to cysteine the aligned positions in the 5-HT3A
and 5-HT3B subunit 1 2 loops K81 and Q70, respectively. The maximal 5-HT-activated currents in receptors containing 5-HT3A /K81C
or 5-HT3B /Q70C were markedly reduced compared with wild type. Desensitization of wild-type currents involved fast and slow compo-
nents. 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
wildtype,K81C,andK81Caftermodificationweresimilar.WetestedtheproximityofK81CtotheM2M3loopbymutatingM2M3loop
residuestocysteineintheK81Cbackground.Disulfidebondsformedin5-HT3A /K81C/A304Cand5-HT3A /K81C/I305Cwhencoexpressed
with5-HT3B.Weconcludethatintherestingstate,K81isnotinahydrophobicpocketassuggestedbythepin-into-sockethypothesis.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, 1 2 loop movement moves M2 and the M2M3 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.
Key words: ion channel; SCAM; acetylcholine; GABA; serotonin; cross-linking

  

Source: Akabas, Myles - Department of Physiology and Biophysics, Albert Einstein College of Medicine, Yeshiva University

 

Collections: Biology and Medicine