The auxiliary subunits Neto1 and Neto2 have distinct, subunit-dependent effects at recombinant GluK1- and GluK2-containing kainate receptors

- Both Neto1 and Neto2 increase glutamate sensitivity of GluK1 receptors.
- Neto1 slows onset of desensitization of GluK1 receptors in response to sub-maximal but not saturating glutamate levels.
- Neto2 slows onset of desensitization of GluK1 and GluK2 receptors at all levels of activation.
- The differing effects of Neto1 and Neto2 at GluK1 receptors are associated with their extracellular CUB domains.

The kainate-type of ionotropic glutamate receptors are assembled from a combination of five different pore-forming subunits (GluK1-5), which confer distinct functional and pharmacological properties. These receptors are also modulated by co-assembly with the auxiliary subunits Neto1 and Neto2. To determine the impact of variation in subunit composition on the functional interaction between kainate receptors and Neto subunits, the Neto subunits were combined with either GluK1 or GluK2 in HEK-293T cells and responses to glutamate examined through patch-clamp recordings. Co-expression of GluK1 with either Neto1 or Neto2 caused a substantial increase in glutamate sensitivity and a slowing of the onset of desensitization at low agonist concentrations. However, at higher glutamate concentrations the primary effect of Neto2 was to slow the onset of desensitization, while that of Neto1 was to increase recovery from desensitization. In contrast, co-expression of Neto2 with GluK2 homomeric receptors had only modest effects on glutamate sensitivity, but increased the rate of recovery from desensitization as well as slowing its onset at all agonist concentrations. The properties of chimeric Neto1/Neto2 subunits suggested that the extracellular N-terminal region including the two CUB domains was largely responsible for the distinct regulatory effects of Neto1 and Neto2 on the desensitization properties of GluK1 homomeric receptors. These results further demonstrate that the functional effects of Neto subunits depend upon the subunit identity of both the auxiliary and the pore-forming subunits.