Transport and Regulation of Polypeptide Precursors of Mature Mitochondrial Proteins

Abstract

Most mitochondrial proteins are coded by nuclear genes, synthesized on cytosolic ribosomes, and then imported into mitochondria. As is the case for some other proteins, most mitochondrial proteins are synthesized as larger precursors. This chapter discusses some of the characteristics and regulatory aspects of protein transport from cytosol to mitochondria. The large majority of mitochondrial proteins are coded by genes of the nucleus, synthesized in the cytoplasmic ribosomes, and then transported to the mitochondria. This seems to be the type of synthesis that takes place for the proteins of the mitochondrial matrix, those of the outer membrane, and most of the proteins of the inner membrane. The enzymes that participate in the replication, transcription, and translation of mitochondrial DNA are among the proteins synthesized in this way. The most widely accepted explanation of why most mitochondrial proteins are synthesized in the cytosol despite having their own genetic system is related to the possible evolutionary origin of this organelle. In this respect, it has been demonstrated recently that in Neurospora crassa , there is a gene in the mitochondrial DNA that codes for a protein homologous to one coded for by nuclear DNA. This suggests and is congruent with the endosymbiotic hypothesis for the origin of eukaryotic mitochondria-gene duplication followed by the transfer of one of these genes to the nucleus. This transfer of DNA between organelles has been extended to the passage of DNA from the chloroplast to the mitochondria.