Anja Schneider - Manganese Transporter in Chloroplasts

Plants, like all other living organism, require manganese (Mn) as an activator of enzymes or as an integral component of protein complexes. The most prominent role of Mn is its involvement in the oxygen evolving complex of photosystem II (PSII), which splits water into oxygen, protons and electrons. The electrons released from water in PSII are eventually transferred to NADP+ via the cytochrome b6/f complex, plastocyanin, photosystem I, and ferredoxin. To reach its destination in the oxygen-evolving complex of PSII in the thylakoid lumen, Mn must cross all three chloroplast membranes (the outer and inner envelopes and the thylakoid membrane). The outer envelope membrane is non-selectively permeable to most ions, while the inner envelope and the thylakoid membrane possess specific transport proteins. At the inner envelope membrane, the CHLOROPLAST MANGANESE TRANSPORTER1 (CMT1) represents the first candidate for the long-sought Mn import protein that transports Mn from the cytosol into the chloroplast stroma. The Arabidopsis mutant cmt1 exhibits severe growth retardation, disruption of chloroplast ultrastructure and reduction of PSII activity, in association with a significant drop in the level of Mn in the organelle. The protein responsible for the transport of Mn from the chloroplast stroma into the thylakoid lumen is PHOTOSYNTHESIS AFFECTED MUTANT71 (PAM71). PAM71 resides in the thylakoid membrane und in pam71 mutants PSII function is specifically impaired at the oxygen-evolving complex. The Mn transport function of CMT1 and PAM71 is conserved among homologous proteins from other oxygenic photosynthetic organisms, e.g. cyanobacteria and green algae and a homologous protein is also found in human. Mn deficiency of pam71 can be rescued if the human protein is targeted to the thylakoid membrane in this mutant, indicating a similar transport function.