AMPA-type glutamate receptors (GluRs) play major roles in excitatory synaptic transmission. Neuronal AMPA receptors comprise GluR subunits and transmembrane AMPA receptor regulatory proteins (TARPs). Previous studies identified five mammalian TARPs, gamma-2 (or stargazin), gamma-3, gamma-4, gamma-7, and gamma-8, that enhance AMPA receptor function. Here, we classify gamma-5 as a distinct class of TARP that modulates specific GluR2-containing AMPA receptors and displays properties entirely dissimilar from canonical TARPs. Gamma-5 increases peak currents and decreases the steady-state currents selectively from GluR2-containing AMPA receptors. Furthermore, gamma-5 increases rates of GluR2 deactivation and desensitization and decreases glutamate potency. Remarkably, all effects of gamma-5 require editing of GluR2 mRNA. Unlike other TARPs, gamma-5 modulates GluR2 without promoting receptor trafficking. We also find that gamma-7 regulation of GluR2 is dictated by mRNA editing. These data establish gamma-5 and gamma-7 as a separate family of "type II TARPs" that impart distinct physiological features to specific AMPA receptors.