Premium
Subunit‐specific desensitization of heteromeric kainate receptors
Author(s) -
Mott David D.,
Rojas Asheebo,
Fisher Janet L.,
Dingledine Raymond J.,
Benveniste Morris
Publication year - 2010
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2009.185207
Subject(s) - homomeric , kainate receptor , ampa receptor , glutamate receptor , agonist , biophysics , xenopus , kainic acid , receptor , chemistry , protein subunit , desensitization (medicine) , patch clamp , biology , biochemistry , gene
Kainate receptor subunits can form functional channels as homomers of GluK1, GluK2 or GluK3, or as heteromeric combinations with each other or incorporating GluK4 or GluK5 subunits. However, GluK4 and GluK5 cannot form functional channels by themselves. Incorporation of GluK4 or GluK5 into a heteromeric complex increases glutamate apparent affinity and also enables receptor activation by the agonist AMPA. Utilizing two‐electrode voltage clamp of Xenopus oocytes injected with cRNA encoding kainate receptor subunits, we have observed that heteromeric channels composed of GluK2/GluK4 and GluK2/GluK5 have steady state concentration–response curves that were bell‐shaped in response to either glutamate or AMPA. By contrast, homomeric GluK2 channels exhibited a monophasic steady state concentration–response curve that simply plateaued at high glutamate concentrations. By fitting several specific Markov models to GluK2/GluK4 heteromeric and GluK2 homomeric concentration–response data, we have determined that: (a) two strikingly different agonist binding affinities exist; (b) the high‐affinity binding site leads to channel opening; and (c) the low‐affinity agonist binding site leads to strong desensitization after agonist binding. Model parameters also approximate the onset and recovery kinetics of desensitization observed for macroscopic currents measured from HEK‐293 cells expressing GluK2 and GluK4 subunits. The GluK2(E738D) mutation lowers the steady state apparent affinity for glutamate by 9000‐fold in comparison to GluK2 homomeric wildtype receptors. When this mutant subunit was expressed with GluK4, the rising phase of the glutamate steady state concentration–response curve overlapped with the wildtype curve, whereas the declining phase was right‐shifted toward lower affinity. Taken together, these data are consistent with a scheme whereby high‐affinity agonist binding to a non‐desensitizing GluK4 subunit opens the heteromeric channel, whereas low‐affinity agonist binding to GluK2 desensitizes the whole channel complex.