Poster: Photosynthesis

Abs # 368: Two P-type ATPses are essential for photosynthetic electron transport in chloroplasts

Presenter:	Abdel-Ghany, Salah , salah@lamar.colostate.edu
Authors	Abdel-Ghany, Salah  (A)   Shikanai, Toshiharu  (B)   Mueller-Moule, Patricia  (C)   Pilon, Marinus  (A)
Affiliations:	(A): Department of Biology, Colorado State University, Ft Collins, CO. USA (B): Graduate School of Biological Science, Nara institute of science, Ikoma, Japan (C): Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102 USA

Copper is a cofactor for enzymes involved in diverse biological processes including photosynthesis. In chloroplasts, copper is a cofactor for the stromal free radical scavenging enzyme copper/zinc superoxide dismutase (CSD2) and for the redox carrier plastocyanin. We studied the function of two transmembrane proteins encoded by PAA1 and PAA2, belonging to the P-type ATPase family of metal transporters. PAA1 is expressed in both root and green tissues, with higher expression level in green tissues. In contrast, PAA2 was present in green tissues but not detected in root tissues by RT-PCR. Mutant alleles of PAA1 and PAA2 show high chlorophyll fluorescence. The high chlorophyll fluorescence phenotype was due to a decrease in both photochemical Electron Transport Rate, (ETR) and Non-Photochemical Quenching (NPQ). Both paa1 and paa2 mutants have a reduced plastocyanin content with the remaining plastocyanin present in the Cu-less inactive form (apo-form). Plastocyanin content could be rescued by the addition of 50 µM copper to the medium in paa1 mutants but not in paa2 mutants. Chloroplastic CSD2 activity was also reduced in paa1 mutants but enhanced in paa2 mutants. The Copper content of both paa1 and paa2 chloroplasts was greatly reduced. Although both paa1 and paa2 mutants are viable, the paa1 x paa2 double mutants are lethal at the early stages of germination. We hypothesize that Paa1 is localized to the chloroplast envelope and delivers copper to the chloroplast; Paa2 is localized to the thylakoid membrane and delivers copper to plastocyanin. Paa1 and Paa2 are critical components of the copper trafficking pathway in chloroplasts-responsible for cofactor delivery to chloroplastic copper proteins and for efficient photosynthetic activity.