Regulates the activity of L-type calcium channels that contain CACNA1C as pore-forming subunit. Regulates the trafficking and gating properties of AMPA-selective glutamate receptors (AMPARs), including GRIA1 and GRIA4. Promotes their targeting to the cell membrane and synapses and modulates their gating properties by slowing their rates of activation, deactivation and desensitization and by mediating their resensitization.