Molecular mechanisms underlying the acquisition of stable electrical phenotypes in developing neurons remain poorly defined. As Xenopus embryonic spinal neurons mature, they initially exhibit dramatic changes in excitability due to a 3-fold increase in voltage-gated-potassium current (I_Kv) density. Later, when mature neurons begin synapse formation, I_Kv density remains stable. Elevation of Kv1.1 and Kv2.1 RNA levels indicates that excess transcript levels of these Kv genes can increase current density in both young and mature neurons. In contrast, Kv2.2 overexpression increases I_Kv density in young but not mature neurons despite the presence of protein translated from injected RNA at this stage. Because protein domains can determine biophysical as well as subcellular localization properties of channel subunits, we tested whether a a region of the Kv2.2 subunit regulated functional expression in mature neurons. We focused on the large cytoplasmic carboxy tail, a region that differs most between Kv2.2 and the structurally-related Kv2.1 subunit. Chimeric Kv2 subunits were generated in which different regions of the large cytoplasmic carboxyl tail were exchanged between Kv2.1 and Kv2.2 subunits. All chimeric Kv2 subunits induced voltage-gated potassium currents when expressed heterologously in oocytes. In vivo, chimeric subunits increased I_Kv density in young neurons upon overexpression in the developing embryo. In contrast, in mature neurons, only those chimerae lacking a domain in the proximal carboxyy terminus, proxC, increased I_Kv density when overexpressed. Thus, the proxC domain mediates developmental and subunit-specific regulation of I_Kv and identifies a novel function for protein domains.