Calmodulin Independent Trafficking of Kv7.2 Channels

Abstract

Helices A and B of the C-terminus of Kv7 channels constitute the calmodulin (CaM) binding site, an auxiliary protein required for exiting the endoplasmic reticulum (ER) and reaching the cell surface. CaM has also been proposed as a regulator of the biosynthesis and assembly of Kv7 subunits. Mutations in this binding site that impair association between Kv7.2 and CaM lead to Benign Familial Neonatal Convulsions (BFNC), a dominantly inherited human epilepsy. However, it is still unclear whether CaM binding to Kv7 is constitutive and required for channel function. Using Tac chimeras, we have identified possible trafficking signals within the CaM binding site of Kv7.2. In addition, we have identified a point mutation (S511D) within the trafficking site of helix B that disrupts CaM binding. In contrast to other CaM-binding disrupting mutations that lead to ER retention, the helix B point mutant is functional and reaches the plasma membrane as efficiently as wild-type channels. In consequence, our data reveals a critical role for helix B in CaM-dependent trafficking, and demonstrates that the constitutive tethering of CaM is not required for Kv7 channel function.