Multiple determinants in voltage-dependent P/Q calcium channels control their retention in the endoplasmic reticulum

Surface expression level of voltage-dependent calcium channels is tightly controlled in neurons to avoid the resulting cell toxicity generally associated with excessive calcium entry. Cell surface expression of high voltage-activated calcium channels requires the association of the pore-forming subunit, Ca-v alpha, with the auxiliary subunit, Ca-v beta. In the absence of this auxiliary subunit, Ca-v alpha is retained in the endoplasmic reticulum (ER) through mechanisms that are still poorly understood. Here, we have investigated, by a quantitative method based on the use of CD8alpha chimeras, the molecular determinants of Ca-v alpha(2.1) that are responsible for the retention, in the absence of auxiliary subunits, of P/Q calcium channels in the ER (referred to here as 'ER retention'). This study demonstrates that the I-II loop of Ca-v alpha(2.1) contains multiple ER-retention determinants beside the beta subunit association domain. In addition, the I-II loop is not the sole domain of calcium channel retention as two regions identified for their ability to interact with the I-II loop, the N- and C-termini of Ca-v alpha(2.1) , also produce ER retention. It is also not an obligatory determinant as, similarly to low-threshold calcium channels, the I-II loop of Ca-v alpha(1.1) does not produce ER retention in COS7 cells. The data presented here suggests that ER retention is suppressed by sequential molecular events that include: (i) a correct folding of Ca-v alpha in order to mask several internal ER-retention determinants and (ii) the association of other proteins, including the Ca-v beta subunit, to suppress the remaining ER-retention determinants.