Evolution of a plant-specific copper chaperone family for chloroplast copper homeostasis

Abstract

Metallochaperones traffic copper (Cu+) from its point of entry at the plasma membrane to its destination. In plants, one destination is the chloroplast, which houses plastocyanin, a Cu-dependent electron transfer protein involved in photosynthesis. We present a previously unidentified Cu + chaperone that evolved early in the plant lineage by an alternative-splicing event of the pre-mRNA encoding the chloroplast P-type ATPase in Arabidopsis 1 (PAA1). In several land plants, recent duplication events created a separate chaperone-encoding gene coincident with loss of alternative splicing. The plant-specific Cu + chaperone delivers Cu + with specificity for PAA1, which is flipped in the envelope relative to prototypical bacterial ATPases, compatible with a role in Cu + import into the stroma and consistent with the canonical catalytic mechanism of these enzymes. The ubiquity of the chaperone suggests conservation of this Cu + -delivery mechanism and provides a unique snapshot into the evolution of a Cu + distribution pathway. We also provide evidence for an interaction between PAA2, the Cu + -ATPase in thylakoids, and the Cu + -chaperone for Cu/Zn superoxide dismutase (CCS), uncovering a Cu + network that has evolved to fine-tune Cu + distribution.