Copper traffic in plants: roles for newly isolated chloroplast proteins

Abstract

Plant cells must acquire copper for photosynthesis as well as other cellular functions. As the site of photosynthesis, chloroplasts must obtain copper from the cytosol and incorporate it into the photosynthetic machinery and other enzymes while avoiding the inherent toxicity of copper ions. Four genes coding for putative copper homeostasis proteins were identified in Arabidopsis. AtCUTA, a homologue of Escherichia coli CUTA, was characterized as coding for a copper binding protein that is upregulated during senescence as well as in plants deficient in chloroplast copper transport. We hypothesize that involved in chloroplast copper homeostasis. Data indicate that AtCUTA expression is likely posttranslationally regulated. CpCOPZ encodes a predicted protein with a conserved copper chaperone-like metal binding domain at the C-terminus. A fusion protein of the predicted CpCOPZ chloroplast transit peptide and a GFP (Green Fluorescent Protein) passenger protein was localized to chloroplasts. CpCCS is a homologue of yeast Lys7, a copper chaperone for copper-zinc superoxide dismutase. A cpCCS:GFP fusion protein was localized to chloroplasts indicating that the protein is likely the specific chaperone to deliver copper to the chloroplastic Cu-Zn superoxide dismutase (CSD2). ATX2 encodes a predicted homologue of ATX1, which delivers copper to the secretory pathway. A fusion protein of the predicted chloroplast transit peptide of ATX2 and GFP was localized to the cytosol, indicating that the protein functions in the cytosol.