A Novel Degradation Signal Derived from Distal C-terminal Frameshift Mutations of KCNQ2 Protein Which Cause Neonatal Epilepsy

Jun Su,Xu Cao,Kewei Wang

doi:10.1074/jbc.m111.287268

Abstract

Benign familial neonatal convulsions is an autosomal-dominant idiopathic form of epilepsy primarily caused by gene mutations of the voltage-gated Kv7.2/KCNQ2/M-channel that exert only partial dominant-negative effects. However, the mechanism underlying the incomplete dominance of channel mutations, which cause epilepsy in infancy, remains unknown. Using mutagenesis and biochemistry combined with electrophysiology, we identified a novel degradation signal derived from distal C-terminal frameshift mutations, which impairs channel function. This degradation signal, transferable to non-channel CD4, can lead to accelerated degradation of mutant proteins through ubiquitin-independent proteasome machinery but does not affect mRNA quantity and protein trafficking. Functional dissection of this signal has revealed a key five-amino acid (RCXRG) motif critical for degradation. Taken together, our findings reveal a mechanism by which proteins that carry this signal are subject to degradation, leading to M-current dysfunction, which causes epilepsy.

Highlights

The mechanism underlying incomplete dominance of KCNQ2 channel mutations, which causes epilepsy, remains unknown
Using mutagenesis and biochemistry combined with electrophysiology, we identified a novel degradation signal derived from distal C-terminal frameshift mutations, which impairs channel function
Our findings demonstrate that Benign familial neonatal convulsions (BFNC) haploinsufficiency is caused by accelerated degradation of channel proteins carrying the degradation signal, and this cellular mechanism brings about impaired channel function and causes neonatal epilepsy

Summary

Background

The mechanism underlying incomplete dominance of KCNQ2 channel mutations, which causes epilepsy, remains unknown. Using mutagenesis and biochemistry combined with electrophysiology, we identified a novel degradation signal derived from distal C-terminal frameshift mutations, which impairs channel function This degradation signal, transferable to non-channel CD4, can lead to accelerated degradation of mutant proteins through ubiquitin-independent proteasome machinery but does not affect mRNA quantity and protein trafficking. Functional dissection of this signal has revealed a key five-amino acid (RCXRG) motif critical for degradation. Using biochemical approaches combined with electrophysiology, we identified a novel degradation signal in the extended segments of the two frameshift mutations This degradation signal contains a critical five-amino acid motif that leads to accelerated degradation of the mutant proteins through the ubiquitin-independent proteasome machinery but does not affect mRNA quantity and protein trafficking. Our findings demonstrate that BFNC haploinsufficiency is caused by accelerated degradation of channel proteins carrying the degradation signal, and this cellular mechanism brings about impaired channel function and causes neonatal epilepsy

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION