Activation of kainate receptor GluK2–Neto2 complex

Abstract

Kainate receptors (KARs) are tetrameric, ligand-gated ion channels of the ionotropic glutamate receptor family that mediate excitatory neurotransmission and modulate neuronal circuits and synaptic plasticity during development of the central nervous system. KARs are implicated in psychiatric and neurological diseases and represent a target of therapeutic intervention. Native KARs form complexes with neuropilin and tolloid-like auxiliary subunits (Neto1 and Neto2), which modulate their function, trafficking and synaptic localization. Here we present structures of rat GluK2 KAR in the apo closed state and in the open states activated by agonist kainate and positive allosteric modulator BPAM344, solved in the presence and absence of Neto2 using time-resolved cryo-electron microscopy. While the binding of Neto2 does not change the behavior of individual or dimeric ligand-binding domains (LBDs) or the ion channel, it prevents tightening of the interface between two LBD dimers during activation and slows the kinetics of deactivation. Our structures illuminate the mechanism of KAR activation and its modulation by Neto2.

Figure 1: Structures of kainate receptor channel GluK2 in the apo closed and open conducting states, both with 2-fold symmetrical ligand-binding domain (LBD) layer. To open the channel, this time we used agonist kainate and positive allosteric modulator BPAM but no Concanavalin A!

Figure 2: Structures of kainate receptor channel GluK2 in complex with auxiliary subunits Neto2, which slow the receptor kinetics, in the apo closed and open conducting states, both with 2-fold symmetrical ligand-binding domain (LBD) layer!

Movie: Interface between two ligand-binding domain (LBD) dimers (A/D and B/C) of kainate receptor GluK2 becomes tight upon transition to the open state, while auxiliary subunit Neto2 prevents this tightening in GluK2-Neto2 complex.

Neto2 prevents LBD layer compression during KAR activation. Shown is a morph between the closed and open conformations of the LBD layer in GluK2 (left, GluK2Apo-2fold versus GluK2KA-BPAM) and GluK2-2xNeto2 (right, GluK2xNeto2Apo versus GluK2xNeto2KA-BPAM). GluK2 LBDs and Neto2 are shown in cartoon representation, while the molecular surface is also shown for the LBDs. Molecules of KA (green) and BPAM (blue) are shown as space-filling models. Note, a similarly loose interface between two LBD dimers (A/D and B/C) in the closed state becomes tight upon transition of GluK2 to the open state, while Neto2 prevents this tightening in GluK2-2xNeto2.