Regulates the gating properties of AMPA-selective glutamate receptors (AMPARs). Modulates their gating properties by accelerating their rates of activation, deactivation and desensitization. Displays subunit-specific AMPA receptor regulation. Shows specificity for GRIA1, GRIA4 and the long isoform of GRIA2. Thought to stabilize the calcium channel in an inactivated (closed) state (By similarity). The L-type calcium channel is composed of five subunits: alpha-1, alpha-2/delta, beta and gamma. Acts as an auxiliary subunit for AMPA-selective glutamate receptors (AMPARs). Found in a complex with GRIA1, GRIA2, GRIA3, GRIA4, CNIH2, CNIH3, CACNG2, CACNG3, CACNG4, CACNG7 and CACNG8. Interacts with GRIA1, GRIA2, GRIA3 and GRIA4 (By similarity). Brain. Enriched in Bergman glia, as well as a variety of neuronal populations including locus coeruleus, olfactory bulb, lateral septal nucleus, interpeduncular nucleus, and the CA2 and rostral/medial CA1 regions of hippocampus. Belongs to the PMP-22/EMP/MP20 family. CACNG subfamily.