8.A.22 The Ca²⁺ Channel Auxiliary Subunit β Types 1-4 (CCA-β) Family

The calcium β auxiliary subunits are cytosolic proteins with no transmembrane segments. Their structures and interaction sites with the α-subunits have been determined (Hanlon and Wallace, 2002; van Petegem et al., 2004). Expression of β-subunits has been demonstrated in muscle, heart, lung and other tissues. β-subunits can associate with the membrane even in the absence of the principal α-subunit through an acidic motif or lipid modification. It has been suggested that β-subunit-dependent modulation of N-type calcium channels via the mitogen-activated protein kinase might regulate release of neurotransmitters (Fitzgerald, 2002). Like other subunits in the calcium channel complex, these β-subunits are alternatively spliced and modified post-translationally. It has been reported that α2δ subunits (TC# 8.A.18) require the presence of β-subunits in order to be functional (Dolphin et al., 1999). Many protein of TC families 8.A.22 and 8.A.24 and others contain PDZ, SH3 and kinase domains involved in signal transduction, often interacting with receptors and transporters. Therefore, these two families share about 400 aas iin common.

In animals, calcium regulates heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca²⁺ in excitable cells are high-voltage activated (HVA) Ca²⁺ channels, Ca_v (Buraei and Yang, 2010). These are plasma membrane proteins composed of several subunits, including α1, α2δ, β, and γ. Although the principal α₁ subunit contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit plays an essential role in regulating the surface expression and gating properties of HVA Ca²⁺ channels. Ca_vβ is also crucial for the modulation of HVA Ca²⁺ channels by G proteins, kinases, and the Ras-related RGK GTPases. Additional proteins modulate HVA Ca²⁺ channels by binding to Ca_vβ, and it may carry out Ca²⁺ channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca_vβ, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca_vβs reveal how they interact with Cavα1 (Buraei and Yang, 2010).

Voltage-gated calcium channels conduct Ca²⁺ ions in response to membrane depolarization. The resulting transient increase in cytoplasmic free calcium concentration is a critical trigger for the initiation of such vital responses as muscle contraction and transcription. L-type Ca_v1.2 calcium channels are complexes of the pore-forming α_1C subunit associated with cytosolic Ca_vβ subunits. All major Ca_vβs share a highly homologous membrane associated guanylate kinase-like (MAGUK) domain that binds to α_1C at the α-interaction domain (AID), a short motif in the linker between transmembrane repeats I and II. In this study we show that Ca_vβ subunits form multimolecular homo- and heterooligomeric complexes in human vascular smooth muscle cells expressing native calcium channels and in Cos7 cells expressing recombinant Ca_v1.2 channel subunits. Ca_vβs oligomerize at the α_1C subunits residing in the plasma membrane and bind to the AID. However, Ca_vβ oligomerization occurs independently on the association with α_1C. Molecular structures responsible for Ca_vβ oligomerization reside in 3 regions of the guanylate kinase subdomain of MAGUK. An augmentation of Ca_vβ homooligomerization significantly increases the calcium current density, while heterooligomerization may also change the voltage-dependence and inactivation kinetics of the channel. Thus, oligomerization of Ca_vβ subunits represents a novel and essential aspect of calcium channel regulation.