Difference between chaperones found in cytosol and those in the mitochondria
Answers
Mitochondria and chloroplasts are surprisingly similar. Both are bounded by two membranes; chloroplasts contain, in addition, an internal membrane compartment — the thylakoids — on which photosynthesis takes place (see Figure 16-34). Both organelles use a proton-motive force and the same type of protein — an F-class ATPase — to synthesize ATP (see Figure 16-2); they also contain similar types of electron-transport proteins. Growth and division of mitochondria and chloroplasts is not coupled to nuclear division. These organelles grow by the incorporation of proteins and lipids, a process that occurs continuously during the interphase period of the cell cycle. As the organelles increase in size, one or more daughters pinch off in a manner similar to the way in which bacterial cells grow and divide. Although the biogenesis of both organelles is similar in many respects, our discussion focuses on mitochondrial biogenesis, about which more is known.
Mitochondria and chloroplasts probably arose by the incorporation of photosynthetic or nonphotosynthetic bacteria into ancestral eukaryotic cells, about 1,500 million years ago, and their subsequent replication in the cytoplasm. Over eons of evolution much of the bacterial DNA in these endosymbionts moved to the nucleus, so that in present-day cells many mitochondrial and chloroplast proteins are imported into the organelles after their synthesis in the cytosol. The mitochondrial and chloroplast DNA found in extant organisms encodes organelle rRNAs and tRNAs but relatively few proteins, mainly subunits of integral membrane proteins essential to organelle function (see Figure 9-44). These proteins are synthesized on ribosomes within the organelles and directed to the correct compartment immediately after synthesis, and current work is elucidating how this happens.
Most chloroplast and mitochondrial proteins, however, are synthesized outside the organelle on cytosolic ribosomes that are not bound to the rough endoplasmic reticulum. The newly made proteins are released into the cytosol and are then taken up specifically into the proper organelle by binding to receptor proteins on the organelle surface that recognize specific uptake-targeting sequences in the new proteins (Table 17-1). The mitochondrion and chloroplast contain multiple membranes and membrane-limited spaces. Thus targeting of some proteins requires the sequential action of two targeting sequences and two membrane-bound receptor systems: one to direct the protein into the organelle, and the other to direct it into the correct organellar compartment or membrane (see Figure 17-1). In general, protein uptake into mitochondria or chloroplasts is an energy-requiring process that depends on integral proteins in the organellar membranes. The biogenesis of both organelles is similar in many respects.