Protein import into chloroplasts requires a chloroplast ATPase.

Abstract

We have transcribed mRNA from a cDNA clone coding for pea ribulose-1,5-bisphosphate carboxylase, translated the mRNA in a wheat germ cell-free system, and studied the energy requirement for posttranslational import of the [35S]methionine-labeled protein into the stroma of pea chloroplasts. We found that import depends on ATP hydrolysis within the stroma. Import is not inhibited when H+, K+, Na+, or divalent cation gradients across the chloroplast membranes are dissipated by ionophores, as long as exogenously added ATP is also present during the import reaction. Our data suggest that protein import into the chloroplast stroma requires a chloroplast ATPase that does not function to generate a membrane potential for driving the import reaction but that exerts its effect in another, yet-to-be-determined, mode. We have carried out a preliminary characterization of this ATPase regarding its nucleotide specificity and the effects of various ATPase inhibitors.