Copper metalloregulation of gene expression

Principles to recover copper-conducting CTR proteins for the purpose of structural and functional studies

Transition metals such as copper and zinc are essential elements required for the survival of most organisms, from bacteria to humans. Yet, elevated levels of these elements are highly toxic. The Copper TRansporter protein family (CTRs) represents the only identified copper uptake proteins in eukaryotes and hence serves as key components for the maintenance of appropriate levels of the metal. Moreover, CTRs have been proposed to serve as an entry point into cells of certain cancer drugs and to constitute attractive drug-targets for novel antifungals. Nevertheless, the structure, function, and regulation of the CTRs remain elusive, limiting valuable information also for applied sciences. To this end, here we report procedures to isolate a range of CTR members using Saccharomyces cerevisiae as a production host, focusing on three homologs, human CTR1, human CTR2, and Candida albicans CTR. Using forms C-terminally-linked to a protease cleavage sequence, Green Fluorescent Protein (GFP), and a His-tag, assessment of the localization, quantification and purification was facilitated. Cellular accumulation of the proteins was investigated via live-cell imaging. Detergents compatible with acceptable solubilization yields were identified and fluorescence-detection size-exclusion-chromatography (F-SEC) revealed preferred membrane extraction conditions for the targets. For purification purposes, the solubilized CTR members were subjected to affinity chromatography and SEC, reaching near homogeneity. The quality and quantity of the CTRs studied will permit downstream efforts to uncover imperative biophysical aspects of these proteins, paving the way for subsequent drug-discovery studies.

The role of post-transcriptional modulators of metalloproteins in response to metal deficiencies

Copper and iron proteins have a wide range of functions in living organisms. Metal assembly into metalloproteins is a complex process, where mismetalation is detrimental and energy consuming to cells. Under metal deficiency, metal distribution is expected to reach a metalation ranking, prioritizing essential versus dispensable metalloproteins, while avoiding interference with other metals and protecting metal-sensitive processes. In this review, we propose that post-transcriptional modulators of metalloprotein mRNA (ModMeR) are good candidates in metal prioritization under metal-limited conditions. ModMeR target high quota or redundant metalloproteins and, by adjusting their synthesis, ModMeR act as internal metal distribution valves. Inappropriate metalation of ModMeR targets could compete with metal delivery to essential metalloproteins and interfere with metal-sensitive processes, such as chloroplastic photosynthesis and mitochondrial respiration. Regulation of ModMeR targets could increase or decrease the metal flow through interconnected pathways in cellular metal distribution, helping to achieve adequate differential metal requirements. Here, we describe and compare ModMeR that function in response to copper and iron deficiencies. Specifically, we describe copper-miRNAs from Arabidopsis thaliana and diverse iron ModMeR from yeast, mammals, and bacteria under copper and iron deficiencies, as well as the influence of oxidative stress. Putative functions derived from their role as ModMeR are also discussed.

Structure and function of copper uptake transporters

Owing to their redox and coordination chemistry copper ions play essential roles in cellular function. Research over the past 20 years has shed much light on the biochemistry of copper homeostasis, and the emergence of high-resolution crystal structures for many of the proteins that partake in cellular copper biology have began to provide insight into the molecular mechanisms by which cells handle this important metal. A notable gap in our understanding is related to the process by which cells acquire copper ions. This chapter describes recent progress in the structure determination of cellular copper uptake transporters and how the emerging structural information aids understanding of the molecular mechanisms that govern cellular copper acquisition and distribution.

The fictile coordination chemistry of cuprous-thiolate sites in copper chaperones

Copper plays vital roles in the active sites of cytochrome oxidase and in several other enzymes essential for human health. Copper is also highly toxic when dysregulated; because of this an elaborate array of accessory proteins have evolved which act as intracellular carriers or chaperones for the copper ions. In most cases chaperones transport cuprous copper. This review discusses some of the chemistry of these copper sites, with a view to some of the structural factors in copper coordination which are important in the biological function of these chaperones. The coordination chemistry and accessible geometries of the cuprous oxidation state are remarkably plastic and we discuss how this may relate to biological function. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

Two Chlamydomonas CTR copper transporters with a novel cys-met motif are localized to the plasma membrane and function in copper assimilation

Inducible high-affinity copper uptake is key to copper homeostasis in Chlamydomonas reinhardtii. We generated cDNAs and updated gene models for four genes, CTR1, CTR2, CTR3, and COPT1, encoding CTR-type copper transporters in Chlamydomonas. The expression of CTR1, CTR2, and CTR3 increases in copper deficient cells and in response to hypoxia or Ni(2+) supplementation; this response depends on the transcriptional activator CRR1. A copper response element was identified by mutational analysis of the 5' upstream region of CTR1. Functional analyses identify CTR1 and CTR2 as the assimilatory transporters of Chlamydomonas based on localization to the plasma membrane and ability to rescue a Saccharomyces cerevisiae mutant defective in high-affinity copper transport. The Chlamydomonas CTRs contain a novel Cys-Met motif (CxxMxxMxxC-x(5/6)-C), which occurs also in homologous proteins in other green algae, amoebae, and pathogenic fungi. CTR3 appears to have arisen by duplication of CTR2, but CTR3 lacks the characteristic transmembrane domains found in the transporters, suggesting that it may be a soluble protein. Thus, Chlamydomonas CTR genes encode a distinct subset of the classical CTR family of Cu(I) transporters and represent new targets of CRR1-dependent signaling.

Three-dimensional structure of the human copper transporter hCTR1

Copper uptake proteins (CTRs), mediate cellular acquisition of the essential metal copper in all eukaryotes. Here, we report the structure of the human CTR1 protein solved by electron crystallography to an in plane resolution of 7 A. Reminiscent of the design of traditional ion channels, trimeric hCTR1 creates a pore that stretches across the membrane bilayer at the interface between the subunits. Assignment of the helices identifies the second transmembrane helix as the key element lining the pore, and reveals how functionally important residues on this helix could participate in Cu(I)-coordination during transport. Aligned with and sealing both ends of the pore, extracellular and intracellular domains of hCTR1 appear to provide additional metal binding sites. Consistent with the existence of distinct metal binding sites, we demonstrate that hCTR1 stably binds 2 Cu(I)-ions through 3-coordinate Cu-S bonds, and that mutations in one of these putative binding sites results in a change of coordination chemistry.

Comparative transcriptome analysis of toxic metal responses in Arabidopsis thaliana and the Cd(2+)-hypertolerant facultative metallophyte Arabidopsis halleri

Toxic effects of both essential and non-essential heavy metals are well documented in plants. Very little is known, however, about their modes of toxicity, about tolerance mechanisms and the signalling cascades involved in mediating transcriptional responses to toxic metal excess. We analysed transcriptome changes upon Cd2+ and Cu2+ exposure in roots of Arabidopsis thaliana and the Cd(2+)-hypertolerant metallophyte Arabidopsis halleri. Particularly, three categories of genes were identified with the help of this comparative approach: (1) common responses, which might indicate stable and functionally relevant changes conserved across plant species; (2) metallophyte-specific responses as well as transcripts differentially regulated between the two species, representing candidate genes for Cd2+ hypertolerance; and (3) those specifically responsive to Cd2+ and therefore indicative of toxicity mechanisms or potentially involved in signalling cascades. Our data define, for instance, Arabidopsis core responses to Cd2+ and Cu2+. In addition, they suggest that Cd2+ exposure very rapidly results in apparent Zn deficiency, and they show the existence of highly specific Cd2+ responses and distinct signalling cascades. Array results were independently confirmed by real-time quantitative PCR, thereby further validating cross-species transcriptome analysis with oligonucleotide microarrays.

Histone H2A and Spt10 cooperate to regulate induction and autoregulation of the CUP1 metallothionein

Copper is an essential cellular cofactor that becomes toxic at high levels. Copper homeostasis is tightly regulated by opposing mechanisms that control copper import, export, and copper binding capacity within the cell. High levels of copper induce the expression of metallothioneins, small sulfhydryl-rich proteins with high metal binding capabilities that serve as neutralizers of toxic levels of metals. In yeast, the CUP1 gene encodes a copper metallothionein that is strongly induced in response to metals and other stress and is subsequently rapidly down-regulated. Activation of CUP1 is mediated by the copper-responsive transcriptional activator AceI, and also requires the histone acetylase Spt10 for full induction. We have examined the role of histone H2A in the normal regulation of the CUP1 gene. We have shown that specific H2A mutations in combination with spt10 deletions result in aberrant regulation of CUP1 expression. Certain lysine mutations in H2A alleviate the transcriptional defect in spt10 Delta strains, though CUP1 activation is still delayed in these mutants; however, CUP1 shutdown is normal. In contrast, serine mutations in H2A prevent CUP1 shutdown when combined with spt10 deletions. In addition, swi/snf mutants exhibit both impaired CUP1 induction and failure to shut down CUP1 normally. Finally, different Spt10-dependent histone acetylation events correlate with induction and shutdown. Taken together, these data indicate that CUP1 transcriptional shutdown, like induction, is an active process controlled by the chromatin structure of the gene. These results provide new insights for the role of chromatin structure in metal homeostasis.