Regulation of high-voltage-activated Ca2+ channel function, trafficking, and membrane stability by auxiliary subunits

Abstract

Voltage-gated Ca²⁺ (Ca_V) channels mediate Ca²⁺ ions influx into cells in response to depolarization of the plasma membrane. They are responsible for initiation of excitation-contraction and excitation-secretion coupling, and the Ca²⁺ that enters cells through this pathway is also important in the regulation of protein phosphorylation, gene transcription, and many other intracellular events. Initial electrophysiological studies divided Ca_V channels into low-voltage-activated (LVA) and high-voltage-activated (HVA) channels. The HVA Ca_V channels were further subdivided into L, N, P/Q, and R-types which are oligomeric protein complexes composed of an ion-conducting Ca_Vα₁ subunit and auxiliary Ca_Vα₂δ, Ca_Vβ, and Ca_Vγ subunits. The functional consequences of the auxiliary subunits include altered functional and pharmacological properties of the channels as well as increased current densities. The latter observation suggests an important role of the auxiliary subunits in membrane trafficking of the Ca_Vα₁ subunit. This includes the mechanisms by which Ca_V channels are targeted to the plasma membrane and to appropriate regions within a given cell. Likewise, the auxiliary subunits seem to participate in the mechanisms that remove Ca_V channels from the plasma membrane for recycling and/or degradation. Diverse studies have provided important clues to the molecular mechanisms involved in the regulation of Ca_V channels by the auxiliary subunits, and the roles that these proteins could possibly play in channel targeting and membrane Stabilization.

FIGURE 1

Subunit composition of high-voltage-activated Ca²⁺ channels and structural domains of the auxiliary subunits. (a) Channel complex is composed of the pore-forming Ca_Vα₁ and the auxiliary Ca_Vα₂δ, Ca_Vβ, and Ca_Vγ subunits. The Ca_Vα₂δ and Ca_Vγ subunits contain transmembrane domains, whereas Ca_Vβs are intracellular. (b) The Ca_Vα₁ subunits consist of four transmembrane domains (I–IV) and the linker joining I and II encompasses the α-interaction domain (AID). (c)The Ca_Vβ subunits are formed by three conserved domains: PSD95/Dlg1/ZO-1 (PDZ), Scr homology 3 (SH3), and guanylate kinase (GK). The Ca_Vβ subunit interaction domain (BID) is one of the regions involved in the interaction of the protein with the Ca_Vα₁ subunit. (d) The Ca_Vα₂δ subunits consist of α₂ (blue), which is an extracellular subunit, disulphide-bonded to the δ subunit (orange), which is membrane-associated. The approximate positions of the vWA domain and the two bacterial chemosensory domains (Cache; C) are also indicated. The sites of interaction between the Ca_Vα₁ subunit and the Ca_Vα₂δ subunit are unknown.

FIGURE 2

Two distinct models for Ca_Vβ subunit-induced increase in Ca_V channel expression at the plasma membrane. (a) In the absence of Ca_Vβ, the Ca_Vα₁ subunit remains trapped within the endoplasmic reticulum (ER) by binding via the I-II loop onto an ER retention protein (ERRP) of unknown identity. Expression and subsequent binding of the Ca_Vβ subunit to the I-II loop relieves the trafficking clamp imposed by the ERRP and allows Ca_V channel complexes to be targeted to the cell surface. (b) In the absence of Ca_Vβ, an ER export signal (blue) present on the Ca_Vα₁ subunit I-II loop is functionally overcome by ER retention signals (pink) present in different regions of the protein, leading to channels being retained in the ER. Upon Ca_Vβ binding to the Ca_Vα₁ subunit, a C-terminus-dependent conformational change of the intracellular domains occurs that diminishes the strength of ER retention signals leading to channel transport to the plasma membrane.

FIGURE 3

Functional coupling between Ca_V channels and secretory vesicles via the Ca_Vβ subunit. (a) RIMs anchor synaptic vesicles next to channels through its interaction with zone-specific proteins (Rab3) and the Ca_Vβ subunit. After depolarization, RIMs regulate the time course of channel inactivation resulting in a sustained Ca²⁺ influx. This molecular organization favors hormone and neurotransmitter release.

FIGURE 4

Hypothetical mechanisms of action of gabapentin (GBP). The effect of GBP may be to displace an endogenous ligand (L-leucine) and impair the ability of the auxiliary Ca_Vα₂δ subunit to increase the number of functional channels at the plasma membrane. GBP may be entering the cells using the system-L transporter protein LAT4 and might be exerting its effect on intracellular Ca_Vα₂δ subunits during assembly and trafficking of the Ca_V channel complex to the cell surface.