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

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Higher plants require chloroplasts for essential functions in photosynthesis and other important physiological processes, such as sugar, lipid and amino-acid biosynthesis. Most chloroplast proteins are nuclear-encoded proteins that are synthesized in the cytosol as precursors, and imported into chloroplasts by protein translocases in the outer and inner chloroplast envelope. The imported chloroplast proteins are then translocated into or across the thylakoid membrane by four distinct pathways. However, the mechanisms by which the imported nuclear-encoded proteins are delivered to these pathways remain largely unknown. Here we show that an Arabidopsis ankyrin protein, LTD (mutation of which causes the light-harvesting chlorophyll-binding protein translocation defect), is localized in the chloroplast and using yeast two-hybrid screens demonstrate that LTD interacts with both proteins from the signal recognition particle (SRP) pathway and the inner chloroplast envelope. Our study shows that LTD is essential for the import of light-harvesting chlorophyll-binding proteins and subsequent routing of these proteins to the chloroplast SRP-dependent pathway.