The GK domain of the voltage-dependent calcium channel β subunit is essential for binding to the α subunit

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

The Ca2+ channel beta4c subunit interacts with heterochromatin protein 1 via a PXVXL binding motif

The β subunits of voltage-gated Ca(2+) channels are best known for their roles in regulating surface expression and gating of voltage-gated Ca(2+) channel α(1) subunits. Recent evidence, however, indicates that these proteins have a variety of Ca(2+) channel-independent functions. For example, on the molecular level, they regulate gene expression, and on the whole animal level, they regulate early cell movements in zebrafish development. In the present study, an alternatively spliced, truncated β4 subunit (β4c) is identified in the human brain and shown to be highly expressed in nuclei of vestibular neurons. Pull-down assays, nuclear magnetic resonance, and isothermal titration calorimetry demonstrate that the protein interacts with the chromo shadow domain (CSD) of heterochromatin protein 1γ. Site-directed mutagenesis reveals that the primary CSD interaction occurs through a β4c C-terminal PXVXL consensus motif, adding the β4c subunit to a growing PXVXL protein family with epigenetic responsibilities. These proteins have multiple nuclear functions, including transcription regulation (TIF1α) and nucleosome assembly (CAF1). An NMR-based two-site docking model of β4c in complex with dimerized CSD is presented. Possible roles for the interaction are discussed.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, 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(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

RGK GTPase-dependent CaV2.1 Ca2+ channel inhibition is independent of CaVbeta-subunit-induced current potentiation

RGK (Rad-Gem-Rem) GTPases have been described as potent negative regulators of the Ca(2+) influx via high-threshold voltage-activated Ca(2+) channels. Recent work, mostly performed on Ca(V)1.2 Ca(2+) channels, has highlighted the crucial role played by the channel auxiliary Ca(V)beta subunits and identified several GTPase and beta-subunit protein domains involved in this regulation. We now extend these conclusions by producing the first complete characterization of the effects of Gem, Rem, and Rem2 on the neuronal Ca(V)2.1 Ca(2+) channels expressed with Ca(V)beta(1) or Ca(V)beta(2) subunits. Current inhibition is limited to a decrease in amplitude with no modification in the voltage dependence or kinetics of the current. We demonstrate that this inhibition can occur for Ca(V)beta constructs with impaired capacity to induce current potentiation, but that it is lost for Ca(V)beta constructs deleted for their beta-interaction domain. The RGK C-terminal last approximately 80 amino acids are sufficient to allow potent current inhibition and in vivo beta-subunit/Gem interaction. Interestingly, although Gem and Gem carboxy-terminus induce a completely different pattern of beta-subunit cellular localization, they both potently inhibit Ca(V)2.1 channels. These data therefore set the status of neuronal Ca(V)2.1 Ca(2+) channel inhibition by RGK GTPases, emphasizing the role of short amino acid sequences of both proteins in beta-subunit binding and channel inhibition and revealing a new mechanism for channel inhibition.