Single-channel behavior of heteromeric alpha1beta glycine receptors: an attempt to detect a conformational change before the channel opens.
The alpha1beta heteromeric receptors are likely to be the predominant synaptic form of glycine receptors in the adult. Their activation mechanism was investigated by fitting putative mechanisms to single-channel recordings obtained at four glycine concentrations (10-1000 microm) from rat alpha1beta receptors, expressed in human embryonic kidney 293 cells. The adequacy of each mechanism, with its fitted rate constants, was assessed by comparing experimental dwell time distributions, open-shut correlations, and the concentration-open probability (P(open)) curve with the predictions of the model. A good description was obtained only if the mechanism had three glycine binding sites, allowed both partially and fully liganded openings, and predicted the presence of open-shut correlations. A strong feature of the data was the appearance of an increase in binding affinity as more glycine molecules bind, before the channel opens. One interpretation of this positive binding cooperativity is that binding sites interact, each site sensing the state of ligation of the others. An alternative, and novel, explanation is that agonist binding stabilizes a higher affinity form of the receptor that is produced by a conformational change ("flip") that is separate from, and precedes, channel opening. Both the "interaction" scheme and the flip scheme describe our data well, but the latter has fewer free parameters and above all it offers a mechanism for the affinity increase. Distinguishing between the two mechanisms will be important for our understanding of the structural dynamics of activation in the nicotinic superfamily and is important for our understanding of mutations in these receptors.
Burzomato
Valeria
V
Department of Pharmacology, University College London, London WC1E 6BT, United Kingdom.
Beato
Marco
M
Groot-Kormelink
Paul J
PJ
Colquhoun
David
D
Sivilotti
Lucia G
LG
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D000870:Anterior Horn Cells / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D001665:Binding Sites; D002460:Cell Line; D003198:Computer Simulation; D004305:Dose-Response Relationship, Drug; D005998:Glycine / Q000008:administration & dosage / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects; D007668:Kidney; D007700:Kinetics; D016013:Likelihood Functions; D008954:Models, Biological; D046912:Multiprotein Complexes; D018408:Patch-Clamp Techniques; D011485:Protein Binding; D011487:Protein Conformation / Q000187:drug effects; D025941:Protein Interaction Mapping; D055503:Protein Multimerization; D051381:Rats; D018009:Receptors, Glycine / Q000819:agonists / Q000737:chemistry* / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism
C491948:Glrb protein, rat; D046912:Multiprotein Complexes; D018009:Receptors, Glycine; D011993:Recombinant Fusion Proteins; D005998:Glycine