A MATRIX-LOCATED PROCESSING PEPTIDASE OF PLANT-MITOCHONDRIA

Nuclear-encoded mitochondrial precursor proteins are proteolytically p rocessed inside the mitochondrion after import. The general mitochondr ial processing activity in plant mitochondria has been shown to be int egrated into the cytochrome bc(1) complex of the respiratory chain. He re we investigate the occurrence of an additional, matrix-located proc essing activity by incubation of the precursors of the soybean mitocho ndrial proteins, alternative oxidase, the F(A)d subunit of the ATP syn thetase and the tobacco F-1 beta subunit of the ATP synthase, with the membrane and soluble components of mitochondria isolated from soybean cotyledons and spinach leaves. A matrix-located peptidase specificall y processed the precursors to the predicted mature form in a reaction which was sensitive to orthophenanthroline, a characteristic inhibitor of mitochondrial processing peptidase (MPP). The specificity of the m atrix peptidase was illustrated by the inhibition of processing of the alternative oxidase precursor in both soybean and spinach matrix extr acts upon altering a single amino acid residue in the targeting preseq uence (-2 Arg to Gly). Additionally, there was no evidence for general proteolysis of precursor proteins incubated with the matrix. The puri ty of the matrix fractions was ascertained by spectrophotometric and i mmunological analyses. The results demonstrate that there is a specifi c processing activity in the matrix of soybean and spinach in addition to the previously well characterized membrane-bound MPP integrated in to the cytochrome bc(1) complex of the respiratory chain.