Auxiliary subunits keep AMPA receptors compact during activation and desensitization

Abstract

Signal transduction at vertebrate excitatory synapses involves the activity of ionotropic glutamate receptors, including the AMPA (-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptor. Technical advances in cryo-electron microscopy have brought a slew of full-length structures of AMPA receptors, on their own and in combination with auxiliary subunits. These structures illustrate a wide range of conformations, indicating that individual domains might undergo substantial lateral motions during gating, resulting in an open, \"relaxed\" extracellular layer. Here, we used bifunctional methanethiosulfonate cross-linkers to calibrate the conformations found in functional AMPA receptors both in the presence and absence of the auxiliary subunit Stargazin. Our data indicate that AMPA receptors have considerable conformational freedom and can get trapped in stable, relaxed conformations, especially upon long exposures to glutamate. In contrast, Stargazin limits this conformational flexibility. Thus, under synaptic conditions, where brief glutamate exposures and the presence of Stargazin dominate, AMPA receptors are unlikely to adopt very relaxed conformations during gating.