Abstract

Metal cofactor assembly in chloroplasts

Dr Marinus Pilon, Colorado State University

Genomic analyses have indicated that about a third of all proteins require a metal cofactor for activity. Cells must regulate and prioritize the delivery of metal ion cofactors by membrane transport processes to targets in various organelles. The growth environment often limits the availability of metal cofactors. Under limitation cells should therefore prioritize delivery to specific targets and coordinate delivery with apo-protein expression as well as varying metabolic demand. Targets for Cu delivery in plant chloroplasts are plastocyanin in the thylakoids and Cu/Zn-superoxide dismutase (Cu/ZnSOD) in the stroma. PAA1 and PAA2 encode Cu-transporting P-type ATPases. Characterization of paa1 and paa2 mutants showed that the two transporters have distinct functions; whereas both transporters are required for Cu delivery to plastocyanin and efficient electron transport, Cu delivery to the stroma is only inhibited in paa1 but not in paa2 mutants. Localization experiments show that both transporters function in chloroplasts and that the PAA1 protein is in the chloroplast periphery. A paa1 paa2 double mutant was seedling lethal and could not be rescued by Cu feeding. A third component of the Cu delivery system in chloroplasts is CCS , a Cu-chaperone that delivers Cu to Cu/ZnSOD. Next to Cu/ZnSOD, plants have a FeSOD in the chloroplast. Interestingly, Cu availability regulates the activity of the SOD isoforms. At low Cu levels, the FeSOD is active and Cu/ZnSOD and CCS expression is shut off, so that Cu is preferentially targeted to plastocyanin, which is essential in plants. At higher Cu levels, FeSOD expression is shut off saving Fe for other uses, and Cu/Zn SOD becomes a sink for Cu in the stroma. We found a signaling pathway, which senses the Cu that is available to the chloroplast and which mediates the down-regulation of non-essential nuclear-encoded Cu-proteins under Cu limitation.

More details as well as links to his excellent publications can be found at http://rydberg.biology.colostate.edu/epsmitslab/