Transport of proteins into mitochondria and chloroplasts.

TLDR: This review will focus on recent papers dealing with the subcellular locations of the sites of synthesis of mitochondrial and chloroplast proteins and with the transport of these proteins into the respective organelles.

Abstract: It has been well established that mitochondria and chloroplasts are not autonomous organeUes. These organelles are capable of nucleic acid and protein synthesis, but many soluble and membrane proteins that become localized in them are initially synthesized on cytoplasmic ribosomes (cf. references 47 and 122). A fundamental question of considerable current interest is: How is the transport of such proteins through the delimiting membranes of the organelle envelope accomplished? A related concern is whether there are discriminatory mechanisms to ensure the specific incorporation of a product of cytoplasmic protein synthesis into its destined organellar location. Although the problem of transport of proteins into mitochondria and chloroplast has been investigated for more than a decade, most of the early studies employed indirect approaches and yielded results that were equivocal. Only with the recent advances in techniques for rnRNA purification (8, 88), in vitro protein synthesis (92, 110), and isolation of purified mitochondria (89, 121) and chloroplasts (98, 100) with intact outer membranes has it been possible to analyze in vitro transport of proteins into these organelles in a systematic and direct manner. Interest in this area has also been stimulated and encouraged by advances made in the understanding of how secretory proteins are transferred across microsomal membranes (cf. reference 22) and how certain plant and microbial toxins are transported into cells (cf. references 104 and 108). This review will focus on recent papers dealing with the subcellular locations of the sites of synthesis of mitochondrial and chloroplast proteins and with the transport of these proteins into the respective organelles.