A Chromodomain Protein Encoded by the Arabidopsis CAO Gene Is a Plant-Specific Component of the Chloroplast Signal Recognition Particle Pathway That Is Involved in LHCP Targeting

Abstract

A recessive mutation in Arabidopsis, named chaos (for chlorophyll a/b binding protein harvesting-organelle specific; designated gene symbol CAO ), was isolated by using transposon tagging. Characterization of the phenotype of the chaos mutant revealed a specific reduction of pigment binding antenna proteins in the thylakoid membrane. These nuclear-encoded proteins utilize a chloroplast signal recognition particle (cpSRP) system to reach the thylakoid membrane. Both prokaryotes and eukaryotes possess a cytoplasmic SRP containing a 54-kD protein (SRP54) and an RNA. In chloroplasts, the homolog of SRP54 was found to bind a 43-kD protein (cpSRP43) rather than to an RNA. We cloned the CAO gene, which encodes a protein identified as Arabidopsis cpSRP43. The product of the CAO gene does not resemble any protein in the databases, although it contains motifs that are known to mediate protein-protein interactions. These motifs include ankyrin repeats and chromodomains. Therefore, CAO encodes an SRP component that is unique to plants. Surprisingly, the phenotype of the cpSRP43 mutant (i.e., chaos ) differs from that of the Arabidopsis cpSRP54 mutant, suggesting that the functions of the two proteins do not strictly overlap. This difference also suggests that the function of cpSRP43 is most likely restricted to protein targeting into the thylakoid membrane, whereas cpSRP54 may be involved in an additional process(es), such as chloroplast biogenesis, perhaps through chloroplast-ribosomal association with chloroplast ribosomes. INTRODUCTIONHigher plants require chloroplasts for essential functions ranging from photosynthesis to sugar, lipid, and amino acid biosyntheses. Many of the proteins required for the light reactions of photosynthesis, including the chlorophyll-containing antenna proteins, are localized in the thylakoid membrane. Thylakoid proteins are encoded by both the nuclear and chloroplast genomes. Nuclear-encoded proteins destined for the thylakoid membrane are synthesized with a cleavable N-terminal extension (transit peptide) that targets the protein across the envelope membranes into the stroma (reviewed in Cline and Henry, 1996;Kermode, 1996;Lübeck et al., 1997). Multiple mechanisms exist for targeting proteins from the stroma to the thylakoid membrane (reviewed in Cline and Henry, 1996;Klösgen, 1997). These include spontaneous insertion, a chloroplast Sec r -dependent pathway, a chloroplast signal recognition particle (cpSRP) pathway, and a pathway requiring only an electrochemical potential of ⌬ pH across the thylakoid membrane. These pathways can be distinguished by specific requirements for stromal proteins and/or by energetic requirements when in 1 These authors contributed equally to this work. 2 To whom correspondence should be addressed: E-mail lnussaume @cea.fr; fax 33-442-25-4656. 88The Plant Cell vitro reconstitution assays are performed. In vitro, precursor proteins are targeted exclusively via one of the aforementioned pathways.LHCIIb proteins are the most abundant of the thylakoid membrane ...

LTD is a protein required for sorting light-harvesting chlorophyll-binding proteins to the chloroplast SRP pathway

“…cpSRP54 was distributed almost equally between the stroma and thylakoids, and cpFtsY was mainly associated with the thylakoid membrane (Fig. 2b), which is consistent with previous reports 16,23,34,35 .…”

“…To elucidate the LTD function, we compared the effects of LTD inactivation on LHCP biogenesis with Arabidopsis mutants known to have defects in LHCP biogenesis (chaos, which lacks cpSRP43; ffc, which lacks cpSRP54; and cpftsY, which lacks cpFtsY) 16,23,28 (Fig. 1d).…”

“…Input, 4% of the prey protein. pigment deficiency phenotypes 16,21,23,28,29,34 . The ltd mutant did not develop any fertile flowers (Fig.…”

“…Several Arabidopsis ankyrin proteins have been shown to interact with substrate proteins and participate in the protein translocation process 16,46 . cpSRP43 contains a chromodomain at the N terminus, which is followed by four ankyrin repeats and then two chromodomains 16 .…”

“…LHCP form a soluble transit complex with cpSRP, which consists of cpSRP54 and cpSRP43, after import into the chloroplast [15][16][17] . The ankyrin domain of cpSRP43 interacts with a conserved L18 region between the second and third transmembrane domains of LHCP 18,19 .…”