The Ca(V)β Subunit Protects the I-II Loop of the Voltage-gated Calcium Channel Ca(V)2.2 from Proteasomal Degradation but Not Oligoubiquitination

Ca(V)β subunits interact with the voltage-gated calcium channel Ca(V)2.2 on a site in the intracellular loop between domains I and II (the I-II loop). This interaction influences the biophysical properties of the channel and leads to an increase in its trafficking to the plasma membrane. We have shown previously that a mutant Ca(V)2.2 channel that is unable to bind Ca(V)β subunits (Ca(V)2.2 W391A) was rapidly degraded (Waithe, D., Ferron, L., Page, K. M., Chaggar, K., and Dolphin, A. C. (2011) J. Biol. Chem. 286, 9598–9611). Here we show that, in the absence of Ca(V)β subunits, a construct consisting of the I-II loop of Ca(V)2.2 was directly ubiquitinated and degraded by the proteasome system. Ubiquitination could be prevented by mutation of all 12 lysine residues in the I-II loop to arginines. Including a palmitoylation motif at the N terminus of Ca(V)2.2 I-II loop was insufficient to target it to the plasma membrane in the absence of Ca(V)β subunits even when proteasomal degradation was inhibited with MG132 or ubiquitination was prevented by the lysine-to-arginine mutations. In the presence of Ca(V)β subunit, the palmitoylated Ca(V)2.2 I-II loop was protected from degradation, although oligoubiquitination could still occur, and was efficiently trafficked to the plasma membrane. We propose that targeting to the plasma membrane requires a conformational change in the I-II loop that is induced by binding of the Ca(V)β subunit.