Regulates the trafficking and gating properties of AMPA-selective glutamate receptors (AMPARs). Promotes their targeting to the cell membrane and synapses and modulates their gating properties by regulating their rates of activation, deactivation and desensitization. Acts as an auxiliary subunit for AMPA-selective glutamate receptors (AMPARs). Found in a complex with GRIA1, GRIA2, GRIA3, GRIA4, CNIH2, CACNG2, CACNG3, CACNG4, CACNG5, CACNG7 and CACNG8. Also localizes to the cell membrane of extrasynaptic sites (dendritic shafts, spines of pyramidal cells). Brain. Expressed in the neocortex, hippocampal formation, and cerebellum (at protein level). Belongs to the cornichon family.