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

A recessive mutation in Arabidopsis, named chaos (for chlorophyll a/b bindi ng 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 p roteins in the thylakoid membrane. These nuclear-encoded proteins utilize a chloroplast signal recognition particle (cpSRP) system to reach the thylak oid membrane. Both prokaryotes and eukaryotes possess a cytoplasmic SRP con taining 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, W e cloned the CAO gene, which encodes a protein identified as Arabidopsis cp SRP43. The product of the CAO gene does not resemble any protein in the dat abases, although it contains motifs that are known to mediate protein-prote in interactions. These motifs include ankyrin repeats and chromodomains. Th erefore, CAO encodes an SRP component that is unique to plants. Surprisingl y, the phenotype of the cpSRP43 mutant (i.e., chaos) differs from that of t he Arabidopsis cpSRP54 mutant, suggesting that the functions of the two pro teins do not strictly overlap. This difference also suggests that the funct ion of cpSRP43 is most likely restricted to protein targeting into the thyl akoid membrane, whereas cpSRP54 may be involved in an additional process(es ), such as chloroplast biogenesis, perhaps through chloroplast-ribosomal as sociation with chloroplast ribosomes.