The I-II Loop of the Ca 2+ Channel α 1 Subunit Contains an Endoplasmic Reticulum Retention Signal Antagonized by the β Subunit

Abstract

The auxiliary beta subunit is essential for functional expression of high voltage-activated Ca2+ channels. This effect is partly mediated by a facilitation of the intracellular trafficking of alpha1 subunit toward the plasma membrane. Here, we demonstrate that the I-II loop of the alpha1 subunit contains an endoplasmic reticulum (ER) retention signal that severely restricts the plasma membrane incorporation of alpha1 subunit. Coimmunolabeling reveals that the I-II loop restricts expression of a chimera CD8-I-II protein to the ER. The beta subunit reverses the inhibition imposed by the retention signal. Extensive deletion of this retention signal in full-length alpha1 subunit facilitates the cell surface expression of the channel in the absence of beta subunit. Our data suggest that the beta subunit favors Ca2+ channel plasma membrane expression by inhibiting an expression brake contained in beta-binding alpha1 sequences.

The RGK family of GTP-binding proteins: Regulators of voltage-dependent calcium channels and cytoskeleton remodeling

“…3A). RGK-mediated Ca V β sequestration would result in the retention of newly synthesized Ca V α 1 subunits in an intracellular compartment [52], resulting in a chronic reduction in surface expressed Ca 2+ channels [74]. Subsequent studies have supported this observation [18,44,75], and extended it to demonstrate inhibition of epitope-tagged Ca V α 1 trafficking by co-expressed Rad, Rem and Rem2, as detected by immunofluorescence microscopy [18,41,43].…”

An NMDA Receptor ER Retention Signal Regulated by Phosphorylation and Alternative Splicing

“…ER exit serves as an important checkpoint both in coordinating the sequential assembly of multisubunit protein complexes within the ER and in defining the number of receptors expressed at the plasma membrane (Blount et al, 1990;Green and Claudio, 1993;Brodsky and McCracken, 1999;Ellgaard et al, 1999;Zerangue et al, 1999;Bichet et al, 2000;Margeta-Mitrovic et al, 2000). Here we have provided a first description of ER quality control mechanisms that regulate the plasma membrane delivery of NMDA receptors.…”

“…In many cases, the masking of specific ER retention/ retrieval motifs during receptor assembly regulates the forward trafficking of ion channels and other membrane proteins through the secretory pathway (Zerangue et al, 1999;Bichet et al, 2000;Margeta-Mitrovic et al, 2000). For example, RXR-type ER retention motifs found in each of the ATP-sensitive potassium channel (K ATP ) subunits Kir6.1, Kir6.2, and SUR1 are sequentially masked during subunit oligomerization, thus assuring that only correctly assembled channels reach the plasma membrane (Zerangue et al, 1999).…”

“…A similar RXR motif present in GABA B receptor GB1 subunits is masked by assembly with GB2, ensuring heterodimerization (Margeta-Mitrovic et al, 2000). Also, surface expression of Ca 2ϩ channels is facilitated by ␤ subunits, which mask ER retention domains in ␣ subunits (Bichet et al, 2000). Like these membrane proteins, NMDA receptors possess intracellular regulatory domains that influence intracellular trafficking of mature channels.…”

CaV2 channel subtype expression in rat sympathetic neurons is selectively regulated by α2δ subunits

“…10,11 Functionally, the b subunit associates with the a1 subunit at the endoplasmic reticulum to ensure proper folding of the channel complex, allowing the complex to be trafficked through the secretory pathway. 12,13 The role of a 2 d is more complex. Co-expression of a 2 d potentiates surface expression of a1/b channel complex in most systems.…”