Ubiquitin Ligase RNF138 Promotes Episodic Ataxia Type 2-Associated Aberrant Degradation of Human Ca2.1 (P/Q-Type) Calcium Channels.

Voltage-gated Ca2.1 channels comprise a pore-forming α subunit with auxiliary αδ and β subunits. Ca2.1 channels play an essential role in regulating synaptic signaling. Mutations in the human gene encoding the Ca2.1 subunit are associated with the cerebellar disease episodic ataxia type 2 (EA2). Several EA2-causing mutants exhibit impaired protein stability and exert dominant-negative suppression of Ca2.1 wild-type (WT) protein expression via aberrant proteasomal degradation. Here, we set out to delineate the protein degradation mechanism of human Ca2.1 subunit by identifying RNF138, an E3 ubiquitin ligase, as a novel Ca2.1-binding partner. In neurons, RNF138 and Ca2.1 coexist in the same protein complex and display notable subcellular colocalization at presynaptic and postsynaptic regions. Overexpression of RNF138 promotes polyubiquitination and accelerates protein turnover of Ca2.1. Disrupting endogenous RNF138 function with a mutant (RNF138-H36E) or shRNA infection significantly upregulates the Ca2.1 protein level and enhances Ca2.1 protein stability. Disrupting endogenous RNF138 function also effectively rescues the defective protein expression of EA2 mutants, as well as fully reversing EA2 mutant-induced excessive proteasomal degradation of Ca2.1 WT subunits. RNF138-H36E coexpression only partially restores the dominant-negative effect of EA2 mutants on Ca2.1 WT functional expression, which can be attributed to defective membrane trafficking of Ca2.1 WT in the presence of EA2 mutants. We propose that RNF138 plays a critical role in the homeostatic regulation of Ca2.1 protein level and functional expression and that RNF138 serves as the primary E3 ubiquitin ligase promoting EA2-associated aberrant degradation of human Ca2.1 subunits. Loss-of-function mutations in the human Ca2.1 subunit are linked to episodic ataxia type 2 (EA2), a dominantly inherited disease characterized by paroxysmal attacks of ataxia and nystagmus. EA2-causing mutants may exert dominant-negative effects on the Ca2.1 wild-type subunit via aberrant proteasomal degradation. The molecular nature of the Ca2.1 ubiquitin-proteasome degradation pathway is currently unknown. The present study reports the first identification of an E3 ubiquitin ligase for Ca2.1, RNF138. Ca2.1 protein stability is dynamically regulated by RNF138 and auxiliary αδ and β subunits. We provide a proof of concept that protecting the human Ca2.1 subunit from excessive proteasomal degradation with specific interruption of endogenous RNF138 function may partially contribute to the future development of a novel therapeutic strategy for EA2 patients.