HMA1, a New Cu-ATPase of the Chloro plast Envelope, Is Essential for Growth under Adverse Light Conditions

Abstract

Although ions play important roles in the cell and chloroplast metabolism, little is known about ion transport across the chloroplast envelope. Using a proteomic approach specifically targeted to the Arabidopsis chloroplast envelope, we have identified HMA1, which belongs to the metal-transporting P 1B -type ATPases family. HMA1 is mainly expressed in green tissues, and we validated its chloroplast envelope localization. Yeast expression experiments demonstrated that HMA1 is involved in copper homeostasis and that deletion of its N-terminal His-domain partially affects the metal transport. Characterization of hma1 Arabidopsis mutants revealed a lower chloroplast copper content and a diminution of the total chloroplast superoxide dismutase activity. No effect was observed on the plastocyanin content in these lines. The hma1 insertional mutants grew like WT plants in standard condition but presented a photosensitivity phenotype under high light. Finally, direct biochemical ATPase assays performed on purified chloroplast envelope membranes showed that the ATPase activity of HMA1 is specifically stimulated by copper. Our results demonstrate that HMA1 offers an additional way to the previously characterized chloroplast envelope Cu-ATPase PAA1 to import copper in the chloroplast.Chloroplasts contain a large variety of ions among which metal ions such as copper, iron, manganese, and zinc that are essential for their development and function. Copper is an essential redox cofactor required for a wide variety of processes, including photosynthetic electron transfer reactions (plastocyanin) and detoxification of superoxide radicals (Cu/Zn-superoxide dismutase, SOD) 4 (1). Other metal ions are cofactors for several enzymatic reactions: zinc is associated with chloroplast SOD, methionine synthase, carbonic anhydrase; manganese is required for oxygen evolution in photosynthesis; whereas iron is a cofactor of iron SOD and is found in iron-sulfur clusters of cytochrome b 6 f complex, ferredoxin, photosystem I (PSI), and photosystem II (PSII) (2). All these metals are essential micronutrients but are toxic when present in excess (3). To maintain the concentration of metals within physiological limits, cells possess mechanisms that control the uptake, accumulation, trafficking, and also detoxification of metal ions. Little is known about metal transport into chloroplasts. Until now, the sole chloroplast proteins demonstrated as being involved in metal ions transport are PAA1 (4) and very recently PAA2 (5), two P 1B -type ATPases. PAA1, localized into the chloroplast envelope, supplies copper to the chloroplast, whereas PAA2, localized into the thylakoid membrane, delivers copper to the thylakoid lumen. One important result derived from this work is the fact that the disruption of the PAA1 gene does not fully abolish the import of copper into the chloroplast. From this recent observation, Abdel-Ghany and coworkers (5) concluded that an as yet unidentified and additional way to PAA1 must exist to import copper into the chloroplast....

Metal Homeostasis in Cyanobacteria and Chloroplasts. Balancing Benefits and Risks to the Photosynthetic Apparatus

“…2). In Arabidopsis chloroplasts, two P-type ATPases function in the inner membrane and an additional P-type ATPase functions in the thylakoid membrane (AbdelGhany et al, 2005;Seigneurin-Berny et al, 2005). One chloroplast copper chaperone of the CCS family has been identified (Chu et al, 2005; Fig.…”

“…Mg transport pathways have not been elucidated yet. Cu transport is carried out by the CtaA and PacS P-type ATPases in cyanobacteria (Tottey et al, 2005) and by PAA1, HMA1, and PAA2 in plants (Abdel-Ghany et al, 2005;Seigneurin-Berny et al, 2005). Intracellular Cu transport is carried out by the Atx1 in cyanobacteria and by the Cu chaperone for SOD (CCS) in plants (Chu et al, 2005;Tottey et al, 2005).…”

“…Under Cu limitation, several algae and cyanobacteria species induce the production of cytochrome c 6 , which replaces plastocyanin in the photosynthetic electron transfer pathway (Eriksson et al, 2004). In vascular plant chloroplasts, where cytochrome c 6 is absent, Cu limitation results in a decrease in copper-zinc (Cu-Zn) superoxide dismutase (SOD) levels (Seigneurin-Berny et al, 2005). Fe-limited cyanobacteria produce a photosynthetic antenna complex that facilitates excitation transfer to PSI (Kouril et al, 2005), in effect compensating for limiting Fe with abundant Mg.…”

How do plants respond to copper deficiency?

“…This is probably related to the evolutionary event that plastocyanin rather than cytochrome c 6 was fixed as an electron carrier for photosynthetic electron transport of land plants. Plastocyanin is also essential for the oceanic diatom Thalassiosira oceanica, although other coastal species lack plastocyanin and rely on the function of cytochrome c 6 . 20 In the open sea, iron levels are very low and copper is relatively more available.…”

“…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.…”