Specificity of leaf mitochondrial and chloroplast processing systems for nuclear-encoded precursor proteins

The specificity of the mitochondrial and chloroplast processing enzymes for the nuclear-encoded precursor proteins was investigated. Mitochondrial precursor proteins of the Nicotiana plumbaginifolia and the Neurospora crassa subunits of F₁-ATPase and the Neurospora Rieske FeS precursor protein were processed to the correct mature size by matrix extracts isolated from spinach leaves, yeast, rat liver and beef heart. The mitochondrial extracts failed to process chloroplast precursor proteins of the stromal small subunit of ribulose 1,5-bisphosphate carboxylase and the thylakoid 33 kDa protein of the oxygen-evolving complex. Both mitochondrial F₁ precursors were specifically processed by a soluble stromal extract from chloroplasts. However, no processing of the Rieske FeS precursor protein was observed under the same conditions with the chloroplast extract. The cleavage of the mitochondrial F₁ precursors by the chloroplast extract was shown to be sensitive to the metal chelators EDTA and ortho-phenanthroline. The cleavage site of the mitochondrial F₁ precursor by the chloroplast soluble extract appears to be located at the N-terminus.Abbreviations ATPase adenosine triphosphatase - Rieske FeS non-heme iron sulphur protein of the ubiquinol cytochrome c oxidoreductase complex - Rubisco ribulose 1,5-bisphosphate carboxygenase/oxygenase - RMSF phenylmethylsulphonylfluoride - EDTA ethylenediaminetetraacetic acid