Structure
There are five types of kainate receptor subunits, GluR5 (GRIK1), GluR6 (GRIK2), GluR7 (GRIK3), KA1 (GRIK4) and KA2 (GRIK5), which are similar to AMPA and NMDA receptor subunits and can be arranged in different ways to form a tetramer, a four subunit receptor. GluR5-7 can form homomers (ex. a receptor composed entirely of GluR5) and heteromers (ex. a receptor composed of both GluR5 and GluR6), however, KA1 and KA2 can only form functional receptors by combining with one of the GluR5-7 subunits. Since 2009 the kainate receptor subunits have been renamed to correspond with their gene name. Hence GluR5-7 are now GluK1-3 and KA1 and KA2 are GluK4 and GluK5 respectively.
Each KAR subunit begins with a 400 residue extracellular N-terminal domain, which plays a key role in assembly, followed by the 1st segment of the neurotransmitter binding cleft called S1. This segment then passes through the cell membrane, forming the first of three membrane spanning regions called M1. The M2 segment then begins on the cytoplasmic face of the membrane, pushes into the cell membrane about half way, and then dips back out to the cytoplasm. This segment has been termed the "p loop", and as is the case of closely related AMPA receptors, determines the calcium permeability of the receptor. M2 turns into M3, another transmembrane spanning segment which emerges on the extracellular face to complete the neurotransmitter binding site (a portion called S2). M4 begins extracellularly, and passes again through the membrane into the cytoplasm, forming the C-terminal of the protein.
Read more about this topic: Kainate Receptor
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