Auxiliary subunits: essential components of the voltage-gated calcium channel complex
Introduction
Voltage-gated calcium channels are multi-subunit membrane complexes that allow depolarization induced calcium influx into cells [1]. Voltage-gated calcium channels function in excitation–contraction (EC) coupling, excitation-secretion coupling, neurotransmitter release, regulation of gene expression and neuronal migration. Two classes of voltage-gated calcium channels have been described. The first class are high-voltage-gated channels, which are activated by strong depolarization. These are further classified into the P/Q, N, R and L types on the basis of differential biophysical properties and sensitivity to pharmacological agents. Relatively lower depolarization is sufficient to activate the second class of channels, which are known as the T-type channels.
Biochemical purification has revealed that high-voltage-gated calcium channels are composed of four subunits, including α1, α2δ, β and γ 1., 2.••. The α1 subunit forms the pore of the calcium channel. Both spontaneous mutations and targeted deletions of murine α1 subunits have been identified or generated. It is now clear that mutations in these genes underlie some human diseases including episodic ataxia type 2, stationary congenital night blindness, familial hemiplegic migraine and spinocerebellar ataxia type 6. It has also been possible to identify proteins that are associated with different calcium channel complexes in different tissues [3]. However, only three ‘auxiliary’ subunits, α2δ, β and γ that meet the following criteria have been identified. The criteria are (1) existence in purified channel complexes (2) direct interaction with the α1 pore forming subunit (3) capability to directly modulate the biophysical properties and/or trafficking of the α1 subunits and (4) stable association with the α1 subunit.
In this review, we discuss the structural and functional diversity of the auxiliary subunits, spontaneous mutants and targeted mouse models of auxiliary subunits and their implications for human disease.
Section snippets
α2δ subunits
Four genetically distinct α2δ subunits α2δ-1 – α2δ-4, have been described 4., 5., 6.. Each one of these proteins is differentially expressed in various tissues, including skeletal muscle, heart and brain (Table 1). The diversity of each α2δ subunit arises by alternative splicing. At the protein level, all four subunits show conserved glycosylation sites, cysteine residues and predicted hydrophobicity profiles.
Of all the α2δ subunits, α2δ-1 is the most extensively characterized. α2δ is a product
β subunits
Four distinct genes encode the β subunits (β1−β4) and numerous splice variants are known [14]. All four of the genes are expressed in the brain. A distinct isoform of the β1 subunit, the β1a isoform, is a component of the skeletal muscle voltage-gated calcium channel. In addition to their expression in the brain, each β subunit shows differential expression in other tissues (Table 1).
β is the only subunit of the channel that is entirely cytosolic. Some forms, however, including β1b and rat β2a
γ subunits
γ was originally known to only be associated with the skeletal muscle voltage-gated channel complex [33]. However, recent studies with the stargazer mouse revealed the existence of a neuronal γ subunit [34]. Subsequently, several γ subunits have been identified 35., 36., 37., 38.•. The γ1 subunit is most closely related to the γ6 subunit, while γ2, γ3, γ4 and γ8 share significant homology. The expression of the γ subunits shows wide tissue distribution (Table 1), but the γ1 subunit expression
Interaction of auxiliary subunits with other proteins
A few proteins that interact with the auxiliary subunits have been identified.
Gem
Gem is a small Ras related G protein that has been demonstrated to bind with the β subunit [46••]. The protein also binds calcium/calmodulin and inhibits the trafficking of the α1 subunit to the plasma membrane. The binding of the activated calcium/calmodulin to Gem allows a nucleotide exchange (from GDP to GTP). In the GTP bound form, Gem has a high affinity for the β subunit and binding of the GTP bound form of
Implications for human disease
It is clear that mutations or deletions of most of the auxiliary subunits result in a discernable phenotype and a loss of normal function in mouse models (Table 2). This is not surprising, considering the important physiological role of the calcium channels.
Few human diseases that arise from mutations or functional compromise of the auxiliary subunits have been described. Of these, two mutations in the gene encoding β4 have been identified as potential causes of familial epilepsy and ataxia [47]
Conclusions
The auxiliary subunits of voltage-gated calcium channels mediate important physiological functions and the loss or mutation of these subunits can have severe consequences. The availability of mouse mutants, both spontaneous and targeted, will help significantly to enhance our understanding of the physiological and pathophysiological roles of the auxiliary subunits. Further studies on these mouse models should help to elucidate the intricate role of the auxiliary subunits in neuronal
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
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of special interest
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of outstanding interest
Acknowledgements
We would like to thank all the members of the Campbell laboratory for critical reading of the manuscript and discussion. We would also like to thank Christina Gurnett and Ricardo Felix for comments and discussion. J Arikkath was partly funded by a predoctoral fellowship from the American Heart Association. KP Campbell is an investigator of the Howard Hughes Medical Institute.
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