STUDIES ON PROTEIN PROCESSING FOR MEMBRANE-BOUND SPINACH LEAF MITOCHONDRIAL PROCESSING PEPTIDASE INTEGRATED INTO THE CYTOCHROME BC(1) COMPLEX AND THE SOLUBLE RAT-LIVER MATRIX MITOCHONDRIAL PROCESSING PEPTIDASE

The plant mitochondrial processing peptidase (MPP) that catalyses the cleavage of the presequences from precursor proteins during or after p rotein impart is a membrane-bound enzyme that constitutes an integral part of the be, complex of the respiratory chain. In contrast, MPP fro m mammals is soluble in the matrix space and does not form part of the respiratory chain, In the present study, we have compared the substra te specificity of the isolated spinach leaf bc(1)/MPP with rat liver M PP using synthetic signal peptides and different mitochondrial precurs or proteins. Inhibition studies of processing with synthetic peptides showed a similar inhibition pattern for plant and rat MPP activity, A peptide derived from the presequence of rat liver mitochondrial aldehy de dehydrogenase (ALDH) was a potent inhibitor of the spinach and rat MPP. Two nonprocessed signal peptides, rhodanese and linker-deleted AL DH (a form of ALDH that lacks the RGP linker connecting two helices in the presequence) had lower inhibitory effects towards each protease. The signal peptide from thiolase, another nonprocessed protein, had li ttle inhibitory effect on MPP. Peptides derived from presequence of th e plant Nicotiana plumbaginifolia F-1 beta also showed a similar inhib itory pattern with rat MPP as with spinach MPP processing. In-vitro sy nthesised precursors of plant N. plumbaginifolia F-1 beta and rat live r ALDH were cleaved to mature form by both spinach and rat MPP. Howeve r, the efficiency of processing was higher with the homologous precurs or. Linker-deleted ALDH, rhodanese, and thiolase were not processed by the mammalian or plant MPP. However, both forms of MPP cleaved a muta ted form of rhodanese that possesses a typical MPP cleavage motif, RXY S. Addition of the same cleavage motif to thiolase did not result in processing by either MPP. These results show that similar higher-order structural elements upstream from the cleavage site are important for processing by both the membrane-bound plant and the soluble mammalian MPP.