Ubiquitin ligase RNF138 promotes episodic ataxia type 2-associated aberrant degradation of human CaV2.1 (P/Q-type) calcium channels

Voltage-gated Ca(V)2.1 channels comprise a pore-forming alpha(1A) subunit with auxiliary alpha(2)delta and beta subunits. Ca(V)2.1 channels play an essential role in regulating synaptic signaling. Mutations in the human gene encoding the Ca(V)2.1 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 Ca(V)2.1 wild-type (WT) protein expression via aberrant proteasomal degradation. Here, we set out to delineate the protein degradation mechanism of human Ca(V)2.1 subunit by identifying RNF138, an E3 ubiquitin ligase, as a novel Ca(V)2.1-binding partner. In neurons, RNF138 and Ca(V)2.1 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 Ca(V)2.1. Disrupting endogenous RNF138 function with a mutant (RNF138-H36E) or shRNA infection significantly upregulates the Ca(V)2.1 protein level and enhances Ca(V)2.1 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 Ca(V)2.1WTsubunits. RNF138-H36E coexpression only partially restores the dominant-negative effect of EA2 mutants on Ca(V)2.1WTfunctional expression, which can be attributed to defective membrane trafficking of Ca(V)2.1 WT in the presence of EA2 mutants. We propose that RNF138 plays a critical role in the homeostatic regulation of Ca(V)2.1 protein level and functional expression and that RNF138 serves as the primary E3 ubiquitin ligase promoting EA2-associated aberrant degradation of human Ca(V)2.1 subunits.