yeast-luminometric-xenopus-oocyte-electrophysiological-examinations

Yeast Luminometric and <em>Xenopus</em> Oocyte Electrophysiological Examinations of the Molecular Mechanosensitivity of TRPV4

TRPV4 (Transient Receptor Potentials, vanilloid family, type 4) is widely expressed in vertebrate tissues and is activated by several stimuli, including ...

University of Wisconsin – Madison

Voltage-gated Ca2+ channels (VGCCs) mediate the influx of Ca2+ that regulates many cellular events, and mutations in VGCC genes cause serious hereditary diseases in mammals. The yeast Saccharomyces cerevisiae has only one gene encoding the putative pore-forming alpha1 subunit of VGCC, CCH1. Here, we identify a cch1 allele producing a completely nonfunctional Cch1 protein with a Gly1265 to Glu substitution present in the domain III S2-S3 cytoplasmic linker. Comparison of amino acid sequences of this linker among 58 VGCC alpha1 subunits from 17 species reveals that a Gly residue whose position corresponds to that of the Cch1 Gly1265 is completely conserved from yeasts to humans. Systematic amino acid substitution analysis using 10 amino acids with different chemical and structural properties indicates that the Gly1265 is essential for Cch1 function because of the smallest residue volume. Replacement of the Gly959 residue of a rat brain Cav1.2 alpha1 subunit (rbCII), positionally corresponding to the yeast Cch1 Gly1265, with Glu, Ser, Lys, or Ala results in the loss of Ba2+ currents, as revealed by the patch clamp method. These results suggest that the Gly residue in the domain III S2-S3 linker is functionally indispensable from yeasts to mammals. Because the Gly residue has never been studied in any VGCC, these findings provide new insights into the structure-function relationships of VGCCs.

Essential, completely conserved glycine residue in the domain III S2-S3 linker of voltage-gated calcium channel alpha1 subunits in yeast and mammals.

The Cch1 protein of the yeast Saccharomyces cerevisiae is a homologue of the pore-forming alpha1 subunit of mammalian voltage-gated Ca2+ channels (VGCCs), and it constitutes a high-affinity Ca2+-influx system with the Mid1 protein in this organism. Here, we characterized the kinetic property of a putative Cch1-Mid1 Ca2+ channel overexpressed in S. cerevisiae cells, and showed that the L-type VGCC blockers nifedipine and verapamil partially inhibited Cch1-Mid1 activity, but typical P/Q-, N-, R- and T-type VGCC blockers did not inhibit activity. In contrast, a third L-type VGCC blocker, diltiazem, increased Cch1-Mid1 activity. Diltiazem did not increase Ca2+ uptake in the cch1Delta and mid1Delta single mutants and the cch1Delta mid1Delta double mutant, indicating that the diltiazem-induced increase in Ca2+ uptake is completely dependent on Cch1-Mid1. These results suggest that Cch1 is pharmacologically similar to L-type VGCCs, but the interactions between Cch1 and the L-type VGCC blockers are more complicated than expected.

Ion-channel blocker sensitivity of voltage-gated calcium-channel homologue Cch1 in Saccharomyces cerevisiae.

The pore-forming component of voltage-gated calcium channels, alpha(1) subunit, contains four structurally conserved domains (I-IV), each of which contains six transmembrane segments (S1-S6). We have shown previously that a Gly residue in the S2-S3 linker of domain III is completely conserved from yeasts to humans and important for channel activity. The Gly residues in the S2-S3 linkers of domains I and II, which correspond positionally to the Gly in the S2-S3 linker of domain III, are also highly conserved. Here, we investigated the role of the Gly residues in the S2-S3 linkers of domains I and II of Ca(v)1.2. Each of the Gly residues was replaced with Glu or Gln to produce mutant Ca(v)1.2s; G182E, G182Q, G579E, G579Q, and the resulting mutants were transfected into BHK6 cells. Whole-cell patch-clamp recordings showed that current-voltage relationships of the four mutants were the same as those of wild-type Ca(v)1.2. However, G182E and G182Q showed significantly smaller current densities because of mislocalization of the mutant proteins, suggesting that Gly(182) in domain I is involved in the membrane trafficking or surface expression of alpha(1) subunit. On the other hand, G579E showed a slower voltage-dependent current inactivation (VDI) compared to Ca(v)1.2, although G579Q showed a normal VDI, implying that Gly(579) in domain II is involved in the regulation of VDI and that the incorporation of a negative charge alters the VDI kinetics. Our findings indicate that the two conserved Gly residues are important for alpha(1) subunit to become functional.

Role of glycine residues highly conserved in the S2-S3 linkers of domains I and II of voltage-gated calcium channel alpha(1) subunits.

The yeast Saccharomyces cerevisiae CCH1 gene encodes a homologue of the pore-forming α1 subunit of mammalian voltage-gated calcium channels. Cch1 cooperates with Mid1, a candidate for a putative, functional homologue of the mammalian regulatory subunit α2/δ, and is essential for Ca(2+) influx induced by several stimuli. Here, we characterized two mutant alleles of CCH1, CCH1* (or CCH1-star, carrying four point mutations: V49A, N1066D, Y1145H and N1330S) and cch1-2 (formerly designated mid3-2). The product of CCH1* displayed a marked increase in Ca(2+) uptake activity in the presence and absence of α-factor, and its increased activity was still dependent on Mid1. Mutations in CCH1* did not affect its susceptibility to regulation by calcineurin. In addition, not only was the N1066D mutation in the cytoplasmic loop between domains II and III responsible for the increased activity of Cch1*, but also substitution of another negatively charged amino acid Glu for Asn(1066) resulted in a significant increase in the Ca(2+) uptake activity of Cch1. This is the first report of a hyperactive mutation in Cch1. On the other hand, the cch1-2 allele possesses the P1228L mutation located in the extracellular S1-S2 linker of domain III. The Pro(1228) residue is highly conserved from fungi to humans, and the P1228L mutation led to a partial loss in Cch1 function, but did not affect the localization and expression of Cch1. The results extend our understanding of the structure-function relationship and functional regulation of Cch1.

Article	Year
Essential, completely conserved glycine residue in the domain III S2-S3 linker of voltage-gated calcium channel alpha1 subunits in yeast and mammals. The Journal of biological chemistry\| PubMed ID: 17569661	2007
Ion-channel blocker sensitivity of voltage-gated calcium-channel homologue Cch1 in Saccharomyces cerevisiae. Microbiology (Reading, England)\| PubMed ID: 19047745	2008
Role of glycine residues highly conserved in the S2-S3 linkers of domains I and II of voltage-gated calcium channel alpha(1) subunits. Biochimica et biophysica acta\| PubMed ID: 20067760	2010
Hyperactive and hypoactive mutations in Cch1, a yeast homologue of the voltage-gated calcium-channel pore-forming subunit. Microbiology (Reading, England)\| PubMed ID: 23475949	2013

Jinfeng Teng

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