Calcium Flux Through AvGluR1: A Glutamate Receptor with a Potassium Channel Selectivity Sequence

Abstract

A glutamate receptor found in the freshwater bdelloid rotifer Adineta vaga was proposed to be an evolutionary link between prokaryotic and eukaryotic iGluR receptor classes (Janovjak et al., 2011). Like GluR0 and some related bacterial iGluRs, the pore loop of AvGluR1 has a K+ channel selectivity filter sequence, TXVGYG. However, electrophysiological experiments revealed permeability to both Na+ and K+ but with no permeability to calcium (Janovjak et al., 2011). Surprisingly, during the course of experiments in Xenopus oocytes designed to evaluate the ligand binding and gating properties of AvGluR1 (Lomash et al., 2013), we found that following attenuation of desensitization by the lectin concanavalin A, or by AvGluR1 Cys mutants that cross link ligand binding domain dimer assemblies, inward current responses to glutamate had a large biphasic transient component that varied from cell to cell. Several lines of evidence suggest that this is due to calcium influx through AvGluR1, and subsequent activation of a Ca dependent chloride current. (i) When the membrane potential was depolarized, the biphasic response to glutamate reverted to a non desensitizing ‘square’ outward current response. (ii) Substitution of barium for extracellular calcium blocked the transient component of the AvGluR1 inward current response recorded at negative membrane potentials. (iii) Current voltage plots revealed a large depolarizing shift in the reversal potential for AvGluR1 responses to glutamate, when extracellular calcium was replaced by magnesium. Thus, despite the presence of the TXVGYG signature sequence, AvGluR1 is a non selective cation channel with permeability to both monovalent and divalent cations.Janovjak H, Sandoz G, Isacoff EY (2011) Nature Comm 2: 232.Lomash S, Chittori S, Brown P, Mayer ML (2013) Structure 21: 414-425