A MUTANT DEFICIENT IN THE PLASTID LIPID DGD IS DEFECTIVE IN PROTEIN IMPORT INTO CHLOROPLASTS

Most proteins in chloroplasts are encoded by the nuclear genome and sy nthesized in the cytosol with N-terminal extensions called transit pep tides. Transit peptides function as the import signal to chloroplasts. The import process requires several protein components in the envelop e and stroma and also requires the hydrolysis of ATP. Lipids have been implicated in the import process based on theories or experiments wit h in vitro model systems. We show here that chloroplasts isolated from an Arabidopsis mutant deficient in the plastid lipid digalactosyl dia cylglycerol (DGD) were normal in importing a chloroplast outer membran e protein, but were defective in importing precursor proteins targeted to the interior of chloroplasts. The impairment includes the binding, or docking, step of the import process that is supported by 100 mu M ATP.