PAA1, a P-Type ATPase of Arabidopsis, Functions in Copper Transport in Chloroplasts

Abstract

Copper (Cu) is an essential trace element with important roles as a cofactor in many plant functions, including photosynthesis. However, free Cu ions can cause toxicity, necessitating precise Cu delivery systems. Relatively little is known about Cu transport in plant cells, and no components of the Cu transport machinery in chloroplasts have been identified previously. Cu transport into chloroplasts provides the cofactor for the stromal enzyme copper/zinc superoxide dismutase (Cu/ZnSOD) and for the thylakoid lumen protein plastocyanin, which functions in photosynthetic electron transport from the cytochrome b 6 f complex to photosystem I. Here, we characterized six Arabidopsis mutants that are defective in the PAA1 gene, which encodes a member of the metal-transporting P-type ATPase family with a functional N-terminal chloroplast transit peptide. paa1 mutants exhibited a high-chlorophyll-fluorescence phenotype as a result of an impairment of photosynthetic electron transport that could be ascribed to decreased levels of holoplastocyanin. The paa1-1 mutant had a lower chloroplast Cu content, despite having wild-type levels in leaves. The electron transport defect of paa1 mutants was evident on medium containing Ͻ 1 M Cu, but it was suppressed by the addition of 10 M Cu. Chloroplastic Cu/ZnSOD activity also was reduced in paa1 mutants, suggesting that PAA1 mediates Cu transfer across the plastid envelope. Thus, PAA1 is a critical component of a Cu transport system in chloroplasts responsible for cofactor delivery to plastocyanin and Cu/ZnSOD.

AtCCS is a functional homolog of the yeast copper chaperone Ccs1/Lys7

“…Cells were analyzed 16 h after transformation by confocal microscopy. be more oxidized [8]. We hypothesize that for optimal photosynthesis, the chloroplast needs to have a Cu delivery system that balances the activity of lumenal plastocyanin and stromal SOD enzymes under variable metal supply, ensuring that sufficient SOD activity is present to prevent oxidative damage if plastocyanin is present and PSI can be reduced (see Fig.…”

“…The expression and regulation of SOD isoforms in plants has been a topic of a number of studies, which showed that oxidative stress, light, and electron transport activity affect plastidic SOD isoform expression [3,8,20]. It was recently shown that Cu feeding dramatically affects the activity of FeS-OD and Cu/ZnSOD isoforms, whereas the activity of MnSOD is not affected by Cu availability [9].…”

“…The Arabidopsis COPT1 gene and its four homologs encode copper transporters that may allow the entrance of Cu into cells [5][6][7]. Copper is imported into the chloroplast stroma by the P-type ATPase PAA1 and mutations in PAA1 affect both stromal Cu/ZnSOD activity and PC [8]. The Arabidopsis genome encodes a protein with high sequence similarity to PAA1, called PAA2, which functions to move Cu into the thylakoid lumen for delivery to plastocyanin [9].…”

Copper Transport Across Pea Thylakoid Membranes

“…Copper transporting P-type ATPases were identified in the cytoplasmic and thylakoid membrane of the cyanobacterium Synechococcus (Kanamaru et al, 1994;Phung et al, 1994), and a related protein, PAA1, was shown to be involved in Cu 21 transport in chloroplasts of Pisum (Shikanai et al, 2003). The N terminus of PAA1 targets a fusion protein to the chloroplast, and deletion of PAA1 results in decreased chloroplast copper levels and reduced activity of Cu/Zn superoxide dismutase and plastocyanin.…”

How do plants respond to copper deficiency?

“…2 Despite its physiological importance, excess copper is toxic for plants because of its potential participation in the Fenton reaction. To minimize the damage by excess copper and also respond to copper deficiency, higher plants have several strategies, including the regulation of copper uptake in root cells, 3 strict copper trafficking via P-type ATPases and copper chaperones [4][5][6][7][8][9] or regulation of the levels of copper proteins in response to a change in the metal availability. 10,11 In addition plants respond to copper deficiency by expressing the alternative iron proteins which complement the function of copper proteins.…”