Copper is an essential plant micronutrient playing key roles in many cellular processes. In Arabidopsis, approximately 50% of cellular copper is localized in the chloroplast, mainly associated with the copper superoxide dismutase (Cu/Zn SOD) in the stroma, and with the plastocyanin (PC) in the thylakoïds. Since plastocyanin is essential for photosynthesis, the delivery of copper to the chloroplasts is prioritized by the cell.

In Arabidopsis, three P-type ATPases, HMA1, HMA6 and HMA8 are involved in copper transport in the chloroplast. HMA6 located in the envelope is the major copper import system into chloroplasts, supplying copper for the SOD and for the PC via HMA8. A recent study by our team has suggested that HMA1 is an additional way to import copper, essential under light stress condition ^[1].

Until now, data on HMA1, HMA6 and HMA8 functions were based on indirect measures; the absence of biochemical data can be explained by the difficulty to produce high amounts of these membrane proteins in an active form. The CEA-PM project dedicated to "the screening of alternative expression system for membrane proteins" showed that the bacteria Lactococcus lactis is particularly suitable for the production of plant membrane proteins, including the HMAs ^{[2, 3]}.

Using this expression system, researchers in our Laboratory, in collaboration with researchers at the Chemistry and Biology of Metals laboratory, have performed the first biochemical characterization of a copper ATPase from plant, HMA6. Phosphorylation assays from ATP or inorganic phosphate have demonstrated that HMA6 has the main features of P-type ATPases and is a high affinity monovalent copper transporter. ^[4].

This approach, now applied to HMA1 and HMA8 should provide key information about the relative roles of P-type ATPases in metal homeostasis of the chloroplast.