Identification of Three Novel Ca²⁺ Channel γ Subunit Genes Reveals Molecular Diversification by Tandem and Chromosome Duplication

Abstract

Gene duplication is believed to be an important evolutionary mechanism for generating functional diversity within genomes. The accumulated products of ancient duplication events can be readily observed among the genes encoding voltage-dependent Ca2+ ion channels. Ten paralogous genes have been identified that encode isoforms of the α1 subunit, four that encode β subunits, and three that encode α2δ subunits. Until recently, only a single gene encoding a muscle-specific isoform of the Ca2+ channel γ subunit (CACNG1) was known. Expression of a distantly related gene in the brain was subsequently demonstrated upon isolation of the Cacng2 gene, which is mutated in the mouse neurological mutant stargazer (stg). In this study, we sought to identify additional genes that encoded γ subunits. Because gene duplication often generates paralogs that remain in close syntenic proximity (tandem duplication) or are copied onto related daughter chromosomes (chromosome or whole-genome duplication), we hypothesized that the known positions of CACNG1 andCACNG2 could be used to predict the likely locations of additional γ subunit genes. Low-stringency genomic sequence analysis of targeted regions led to the identification of three novel Ca2+ channel γ subunit genes, CACNG3,CACNG4, and CACNG5, on chromosomes 16 and 17. These results demonstrate the value of genome evolution models for the identification of distantly related members of gene families.[The sequence data described in this paper have been submitted to the GenBank data library under accession numbersAF142618–AF142625 and AF148220.]

Biochemical and Biophysical Evidence for γ2 Subunit Association with Neuronal Voltage-activated Ca2+Channels

“…Previous biophysical studies using recombinant Ca 2ϩ channels have indicated an inhibitory role for the ␥ 2 subunit based on changes in the voltage dependence of steady-state inactivation of the channels (3,10). In addition, a recent finding of novel ␥ isoforms (␥ 3 , ␥ 4 , ␥ 5 ) suggests a possibility that a ␥ gene family originated through tandem and chromosome duplication (11). On the other hand, a recent study has suggested a role of the ␥ 2 subunit on the trafficking/clustering of AMPA 1 receptors (12).…”

Stargazin Reduces Desensitization and Slows Deactivation of the AMPA-Type Glutamate Receptors

“…On the other hand, two types of trafficking proteins were reported to interact with all (or at least three) AMPA receptor subunits: Narp, a secreted immediate-early gene product that binds to the extracellular portions of the subunit molecule (O'Brien et al, 1999), and the transmembrane AMPA receptor regulatory proteins (TARPs) stargazin/␥2, ␥3, ␥4, and ␥8 . TARPs are four-transmembrane proteins with a weak homology to the ␥ subunits of muscle voltage-dependent Ca 2ϩ channels (VDCCs) (Burgess et al, 1999;Klugbauer et al, 2000). Strikingly, a spontaneous loss of function of stargazin in a mouse mutant (Letts et al, 1998) resulted in failure of synaptic surface expression of AMPA receptors in cerebellar granule cells, in addition to other phenotypic manifestations (Hashimoto et al, 1999;Chen et al, 2000).…”

Spatial diversity in gene expression for VDCCγ subunit family in developing and adult mouse brains

“…Subsequent studies have revealed six additional subunits and, hence, there are eight members to date in the VDCCγ family (Black and Lennon., 1999;Burgess et al, 1999Burgess et al, , 2001Klugbauer et al, 2000;Chu et al, 2001;Moss et al, 2002). Each γ subunit contains four putative transmembrane domains with intracellularly located N-and C-termini (Chu et al, 2001;Black, 2003).…”