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, Cav (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. Cavβ 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 Cavβ, and it may carry out Ca2+ channel-independent functions, including directly regulating gene transcription. All four subtypes of Cavβ, 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 Cavβs 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 Cav1.2 calcium channels are complexes of the pore-forming α1C subunit associated with cytosolic Cavβ subunits. All major Cavβs share a highly homologous membrane associated guanylate kinase-like (MAGUK) domain that binds to α1C at the α-interaction domain (AID), a short motif in the linker between transmembrane repeats I and II. In this study we show that Cavβ subunits form multimolecular homo- and heterooligomeric complexes in human vascular smooth muscle cells expressing native calcium channels and in Cos7 cells expressing recombinant Cav1.2 channel subunits. Cavβs oligomerize at the α1C subunits residing in the plasma membrane and bind to the AID. However, Cavβ oligomerization occurs independently on the association with α1C. Molecular structures responsible for Cavβ oligomerization reside in 3 regions of the guanylate kinase subdomain of MAGUK. An augmentation of Cavβ homooligomerization significantly increases the calcium current density, while heterooligomerization may also change the voltage-dependence and inactivation kinetics of the channel. Thus, oligomerization of Cavβ subunits represents a novel and essential aspect of calcium channel regulation.