KCNQ2 (Kv7.2) and KCNQ3 (Kv7.3) are the principal subunits underlying the potassium M-current, which exerts a strong control on neuronal excitability. KCNQ3 subunits coassemble with KCNQ2 to form functional heteromeric channels that are specifically transported to the axonal initial segment and nodes of Ranvier. In contrast, there is no evidence for functional homomeric KCNQ3 channels in neurons, and it appears that these are inefficiently trafficked to the plasma membrane. Among eukaryotic potassium channels, the KCNQ3 subunit is unusual because it has an alanine in place of a threonine at the pore inner vestibule, three residues upstream of the GYG signature sequence of the selectivity filter. This residue is critical for the potentiation of the current after heteromerization, but the mechanism is unknown. We report that the presence of this uncommon residue at position 315 has a strong impact on the stability of the homotetramers and on channel trafficking. Wild-type KCNQ3 expressed alone is retained within the endoplasmic reticulum, and this mechanism is overcome by the substitution of threonine for Ala315. KCNQ3 subunits require assembly with KCNQ2 to exit this compartment, whereas KCNQ3-A315T is no longer dependent on KCNQ2 to form channels that are efficiently trafficked to the plasma membrane. The presence of this alanine, therefore, plays an important role in regulating the subunit composition of functional M-channels expressed at the surface of neurons.