Disruption of Assembly/Calmodulin-Binding Coupling and Calmodulin-Dependent Potentiation of Kv7.2 Channels by a Epileptogenic Helix D Mutation

Kv7.2 is a main component of the neuronal M-current, whose density depends on the interaction with calmodulin (CaM). Several observations hint that CaM binding is coupled to Kv7 subunit multimerization, but such important process is not well-understood. Whereas subunit specific assembly of Kv7 channels depends on helix D, CaM binds to helices A and B located 60 residues upstream. By studying the consequences of the helix D L609R mutation found in patients suffering Neonatal Familial Seizures, we confirm that helix D is crucial for C-terminal multimerization. Surprisingly, disruption of helix D-dependent C-terminal multimerization by L609R has no major consequences for Kv7.2 channel electrophysiological properties. FRET results suggest that CaM enhances multimerization of the wt C-terminal domain, but has marginal effects when the helix D mutation is present. The L609R mutation weakens the interaction with CaM and disrupts CaM-dependent Kv7.2 potentiation. These results underscore a complex interconnection between CaM, PIP2 and multimerization on the regulation of homomeric Kv7.2 channels.