The Arabidopsis COPT6 Transport Protein Functions in Copper Distribution Under Copper-Deficient Conditions

Abstract

Copper (Cu), an essential redox active cofactor, participates in fundamental biological processes, but it becomes highly cytotoxic when present in excess. Therefore, living organisms have established suitable mechanisms to balance cellular and systemic Cu levels. An important strategy to maintain Cu homeostasis consists of regulating uptake and mobilization via the conserved family of CTR/COPT Cu transport proteins. In the model plant Arabidopsis thaliana, COPT1 protein mediates root Cu acquisition, whereas COPT5 protein functions in Cu mobilization from intracellular storage organelles. The function of these transporters becomes critical when environmental Cu bioavailability diminishes. However, little is know about the mechanisms that mediate plant Cu distribution. In this report, we present evidence supporting an important role for COPT6 in Arabidopsis Cu distribution. Similarly to COPT1 and COPT2, COPT6 fully complements yeast mutants defective in high-affinity Cu uptake and localizes to the plasma membrane of Arabidopsis cells. Whereas COPT2 mRNA is only up-regulated upon severe Cu deficiency, COPT6 transcript is expressed under Cu excess conditions and displays a more gradual increase in response to decreases in environmental Cu levels. Consistent with COPT6 expression in aerial vascular tissues and reproductive organs, copt6 mutant plants exhibit altered Cu distribution under Cu-deficient conditions, including increased Cu in rosette leaves but reduced Cu levels in seeds. This altered Cu distribution is fully rescued when the wild-type COPT6 gene is reintroduced into the copt6 mutant line. Taken together, these findings highlight the relevance of COPT6 in shoot Cu redistribution when environmental Cu is limited.

Daily rhythmicity of high affinity copper transport

“…Furthermore, analysis of the COPT1, COPT2 and COPT6 promoter regions reveals multiple putative light and circadian cis-regulatory elements. 9,17,18 Some of these motifs are involved in phytochrome regulation of gene expression, 19 which corroborates the reduction of COPT2 transcript in the phy mutants. 15 Such cis-responsive elements could also participate in the regulation of the transcriptional factor SPL7, as they are found in its promoter (Fig.…”

Function and Regulation of the Plant COPT Family of High-Affinity Copper Transport Proteins

“…Whereas aerial COPT6 transcript upregulation by Cu deficiency is fully dependent on SPL7, its regulation in roots seems to be partially independent of the SPL7 transcription factor [12]. We observed that contrary to COPT1 and COPT2, COPT6 expression is also present under Cu-sufficient conditions [48]. Our analyses of endogenous Cu levels have shown that the copt6 knock-out lines do not exhibit a significant defect in total Cu accumulation under either Cu-sufficient or Cu-deficient conditions.…”

“…We observed that, with the exception of COPT4, the ectopic expression of all the Arabidopsis COPT family members expressed in yeast Cu transport mutants stimulates Cu uptake and accumulation [12,13,49]. Regarding yeast growth complementation, COPT1, COPT2, and COPT6 fully rescued the ctr1Δctr3Δ defect under nonfermentable and low Fe conditions, whereas the COPT3 and COPT5 effect was only partial [6,12,13,48]. We observed that COPT4 expression, which does not possess the key methionine residues essential for Cu transport, including the MX 3 M motif, proves toxic to yeast cells [13].…”

“…Recent studies by our and other research groups have shown that a similar division of functions occurs in Arabidopsis COPT proteins. Whereas fusions of COPT1, COPT2, and COPT6 to the green fluorescent protein (GFP) localize at the plasma membrane in plant cells, COPT5 is intracellularly localized [12,[47][48][49][51][52][53] (Figure 2(a)). As indicated above, the Cu-regulated transcription factor SPL7 specifically activates the expression of COPT1, COPT2, and COPT6 genes in response to Cu deficiency, whereas no Cu regulation has been observed for COPT3 and COPT5 genes [9, 12, 13, 47] (Figure 2(b)).…”

“…This outlook paper focuses on our contribution to characterizing the function and regulation of the different members of the conserved CTR/COPT family of high-affinity Cu transporters in the model plant Arabidopsis thaliana. [6,12,13,[46][47][48]. We observed that, with the exception of COPT4, the ectopic expression of all the Arabidopsis COPT family members expressed in yeast Cu transport mutants stimulates Cu uptake and accumulation [12,13,49].…”