Analysis of mechanism for selective translocation of a protein having two subcellular localization signals

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We analyzed organelle targeting signals of rat serine : pyruvate aminotransferase (SPT) which shows dual subcellular distribution in mitochondria (Mt) and peroxisomes (Ps) . We also examined the molecular mechanism of selective targeting into Mt of mitochondrial SPT precursor which contained both the mitochondrial and peroxisomal targeting signals (MTS and PTS) .(1) Analysis of organelle targeting signals --- It was indicated that the entire region of 22 amino acids of MTS was necessary for the mitochondrial targeting function and that, being different from the typical PTS1 of peroxisomal enzymes, PTS of SPT was not restricted to the 3 C-terminal amino acids and required an additional amino acid sequence for PTS function(2) Effect of MTS on the conformation of the protein --- It has been reported that the mitochondrial precursor proteins were imported into Mt as an unfolded protein, whereas the peroxisomal proteins are imported as a folded molecule. It was indicated by protease digestion experiment that the precursor protein having MTS was highly sensitive to the protease compared with the protein having no MTS.(3) Repression of other PTS by MTS of SPT --- To investigate effect of MTS on the function of other PTS, we constructed a chimeric protein consisting of N-terminal half of SPT and C-terminal half of urate oxidase having PTS1 at the C-terminus , and examined the PTS function in an in vitro peroxisomal import system. The result showed that PTS1 of urate oxidase was also repressed by the presence of MTS of SPT.These results suggest that the folded conformation, especially around C-terminal region of the protein, is necessary for expression of the PTS function and that MTS causes unfolding of the protein, repression of PTS and then the selective translocation into Mt of the precursor SPT having both MTS and PTS.