Structural Insights into KChIP4a Modulation of Kv4.3 Inactivation

Abstract

Dynamic inactivation in Kv4 A-type K+ current is critical in regulating neuronal excitability by shaping action potential waveform and duration. Multifunctional auxiliary KChIPs1-4 subunits that share a high homology in the C-terminal core regions exhibit distinctive modulation on inactivation and surface expression of pore-forming Kv4 subunits. How structural differences of KChIPs in determining their functional diversity remain unknown.Here, we describe a crystal structure of KChIP4a resolved at 3.0 A resolution. The KChIP4a structure shows the distinct N-terminal α-helices that differentiate it from other KChIPs. The N-terminal residues of KChIP4a form a long α-helix followed by a short rigid coil that binds to a well-defined hydrophobic pocket formed by the conserved structural components. Structural comparison indicates that the hydrophobic pocket of KChIP4a is similar to that of KChIP1. This hydrophobic pocket has recently been shown to be the same type of binding groove that is recognized by the Kv4.3 N-terminus in KChIP1(ref. 1-2). The core KChIP4a (without its N-terminal α-helices) reveals that the hydrophobic groove can sequester the N-terminus of Kv4.3. Structural observations suggest that N-termini of both Kv4.3 and KChIP4a can competitively interact with the same hydrophobic pocket of KChIP4a. Biochemical experiments showed that competitive binding of Kv4.3 N-terminal peptide to the hydrophobic groove of core KChIP4a causes the release of its N-terminus that suppresses the inactivation of Kv4.3 channels. Electrophysiology confirmed that the N-terminal first α-helix peptide of KChIP4a either by itself or fused to N-terminal truncated Kv4.3 can confer the slow inactivation. We propose that N-terminal binding of Kv4.3 to the core KChIP4a mobilizes the KChIP4a N-terminus which may serve as a slow inactivation gate.