Active Zone Trafficking of CaV2/UNC-2 Channels Is Independent of & beta;/CCB-1 and & alpha;2 & delta;/UNC-36 Subunits

The CaV2 voltage-gated calcium channel is the major conduit of calcium ions necessary for neurotransmitter release at pre synaptic active zones (AZs). The CaV2 channel is a multimeric complex that consists of a pore-forming a1 subunit and two auxiliary b and a2d subunits. Although auxiliary subunits are critical for channel function, whether they are required for a1 trafficking is unresolved. Using endogenously fluorescent protein-tagged CaV2 channel subunits in Caenorhabditis elegans, we show that UNC-2/a1 localizes to AZs even in the absence of CCB-1/b or UNC-36/a2d, albeit at low levels. When UNC-2 is manipulated to be trapped in the endoplasmic reticulum (ER), CCB-1 and UNC-36 fail to colocalize with UNC-2 in the ER, indicating that they do not coassemble with UNC-2 in the ER. Moreover, blocking ER-associated degradation does not further increase presynaptic UNC-2 channels in ccb-1 or unc-36 mutants, indicating that UNC-2 levels are not regulated in the ER. An unc-2 mutant lacking C-terminal AZ protein interaction sites with intact auxiliary subunit binding sites displays persistent presynaptic UNC-2 localization and a prominent increase of UNC-2 channels in nonsynaptic axonal regions, underscoring a protective role of auxiliary subunits against UNC-2 degradation. In the absence of UNC-2, presynaptic CCB-1 and UNC-36 are profoundly diminished to barely detectable levels, indicating that UNC-2 is required for the presynaptic localization of CCB-1 and UNC-36. Together, our findings demonstrate that although the pore-forming subunit does not require auxiliary subunits for its trafficking and transport to AZs, it recruits auxiliary subunits to stabilize and expand calcium channel signalosomes.