Copper Delivery to Chloroplast Proteins and its Regulation

Abstract

Copper is required for photosynthesis in chloroplasts of plants because it is a cofactor of plastocyanin, an essential electron carrier in the thylakoid lumen. Other chloroplast copper proteins are copper/zinc superoxide dismutase and polyphenol oxidase, but these proteins seem to be dispensable under conditions of low copper supply when transcripts for these proteins undergo microRNA-mediated down regulation. Two ATP-driven copper transporters function in tandem to deliver copper to chloroplast compartments. This review seeks to summarize the mechanisms of copper delivery to chloroplast proteins and its regulation. We also delineate some of the unanswered questions that still remain in this field.

Keywords: Cu-miRNA; copper deficiency; copper transporting P-type ATPase; photosynthesis; plastocyanin; polyphenol oxidase; superoxide dismutase.

FIGURE 1

Chloroplast copper proteins and delivery systems. CSD2 (copper/zinc superoxide dismutase) is active in the stroma and receives Cu ions from CCS (the copper chaperone for superoxide dismutase). PC is active in the thylakoid lumen. PPO is found in the thylakoid lumen but may receive its Cu ion in the stroma before entering the thylakoid space via the TAT protein translocation pathway. PAA1 is a copper transporter in the envelope required to supply Cu ions to CSD2, PC and presumably also PPO. PAA1 receives Cu ions from the plastid copper chaperone 1 (PCH1). Cu ions may reach PCH1 in the inter membrane space of the envelope via diffusion through porins in the outer membrane, perhaps bound to low molecular weight chelators such as glutathione. Alternatively, PCH1 might pick up Cu ions in the cytosol and diffuse in via pores in the outer membrane. PAA2 is a homologs copper transporter in the thylakoid membrane required to deliver Cu ions to PC. A protein donating Cu ions to PAA2 has not been identified yet.

FIGURE 2

Topology model of Cu+ transporting P-type ATPases such as PAA1 and PAA2. The proteins have eight transmembrane segments. Major domains and sequence motifs are indicated. The region that corresponds to the copper chaperone PCH1, encoded in Arabidopsis by a PAA1 alternative splice form, is indicated in a box. The box on the right gives a simplified depiction of the protein in the same topology, which is used in Figure 3 also.

FIGURE 3

Model for the evolution of chloroplast Cu transport from a hypothetical cyanobacterial ancestor of the chloroplast. Cyanobacteria, such as Synechocystis, contain two major Cu proteins, PC in the thylakoids and CCO in both the thylakoids and inner membrane of the envelope. In these cyanobacteria two Cu-transporting P-type ATPases called CtaA and PacS are present together with a copper chaperone ScATX that can interact with both transporters. ScATX serves to sequester Cu ions that entered the cytosol by low affinity pathways and to deliver the Cu ions to CtaA and PacS. PacS is in the thylakoid membranes and loss of function affects both Cu tolerance and PC maturation. CtaA is presumably present in the inner membrane and maybe in addition in an internal vesicle. We propose that CtaA has the indicated topology, which allows the protein to deliver Cu to the exoplasmic side of membranes to allow maturation of both the CCO complex and PC. If CtaA is active in an internal vesicle then Cu ions can reach CCO and PC via a vesicular transport mechanism involving membrane fusion. After endosymbiosis gave rise to the chloroplast, both PacS and ScATX were lost. Nucleus encoded PAA1 and PAA2 then evolved from CtaA with the addition of a transit peptide for chloroplast targeting and for PAA1 also a poly-glycine stretch to ensure envelope retention and correct (novel) topology. In addition, alternative splicing or gene duplication allowed the evolution of the novel Cu chaperone PCH1 from PAA1/CtaA. In analogy to the use of the N-terminus of PAA1 as a copper chaperone, PAA2 may use its N-terminal region as a Cu chaperone in the stroma (CuChap, hypothetical at present) that might be derived from the full transporter via proteolytic processing. Alternatively PAA2 may receive Cu ions that bound to low molecular weight chelators in the stroma or from CCS protein (not shown).