Abstract
Kv channel-interacting proteins (KChIPs) are auxiliary subunits of the heteromultimeric channel complexes that underlie neuronal I(SA), the subthreshold transient K(+) current that dynamically regulates membrane excitability, action potential firing properties, and long term potentiation. KChIPs form cytoplasmic associations with the principal pore-forming Kv4 subunits and typically mediate enhanced surface expression and accelerated recovery from depolarization-induced inactivation. An exception is KChIP4a, which dramatically suppresses Kv4 inactivation while promoting neither surface expression nor recovery. These unusual properties are attributed to the effects of a K channel inactivation suppressor domain (KISD) encoded within the variable N terminus of KChIP4a. Here, we have functionally and biochemically characterized two brain KChIP isoforms, KChIP2x and KChIP3x (also known as KChIP3b) and show that they also contain a functional KISD. Like KChIP4a and in contrast with non-KISD-containing KChIPs, both KChIP2x and KChIP3x strongly suppress inactivation and slow activation and inhibit the typical increases in surface expression of Kv4.2 channels. We then examined the properties of the KISD to determine potential mechanisms for its action. Subcellular fractionation shows that KChIP4a, KChIP2x, and KChIP3x are highly associated with the membrane fraction. Fluorescent confocal imaging of enhanced green fluorescent proteins (eGFP) N-terminally fused with KISD in HEK293T cells indicates that KISDs of KChIP4a, KChIP2x, and KChIP3x all autonomously target eGFP to intracellular membranes. Cell surface biotinylation experiments on KChIP4a indicate that the N terminus is exposed extracellularly, consistent with a transmembrane KISD. In summary, KChIP4a, KChIP2x, and KChIP3x comprise a novel class of KChIP isoforms characterized by an unusual transmembrane domain at their N termini that modulates Kv4 channel gating and trafficking.