Mitochondrial and chloroplast proteins:

Mitochondria and chloroplasts hold their own DNA, mRNA, ribosomes and many more and carry out protein synthesis but very few chloroplast or mitochondrial proteins are build in this way. Rather, the huge majority of mitochondrial and chloroplast proteins are encoded through the nuclear genome are synthesized in the cytosol on free ribosomes and released follows synthesis and then imported into the organelle. Therefore, the procedure is post-translational. The protein may required to be goaled to any one of various locations; for mitochondria this could be the inner membrane, the outer mitochondrial membrane, the mitochondrial matrix or the intermembrane space. Chloroplasts have the similar subcompartments plus two other potential destinations, the thylakoid membrane, the thylakoid space. Most is known about mitochondrial protein uptake. The Proteins are goaled to the mitochondrial matrix through an N-terminal matrix- targeting sequence. This sequence is classically 15-35 amino acids long and rich in hydrophobic amino acids and the hydroxylated amino acids threonine and serine and the positively charged amino acids arginine and lysine. These matrix- targeting sequences possibly suppose an -helical conformation with the positively charged amino acids on one side of the helix and the hydrophobic amino acids on the other, like that these sequences are amphipathic. Following synthesis through cytosolic ribosomes and protein is released into the cytosol but is kept in an unfolded state through chaperone proteins known as the hsp70 family of proteins that bind to it during synthesis. This is must since folded proteins cannot be imported into mitochondria. hsp70 then transfers the unfolded protein to an import receptor in the outer mitochondrial membrane which is believed to slide along the membrane until it reaches a site where the inner membrane and outer membrane are in contact. At this point it passes into the matrix through a protein translocator formed from the components of both membranes shown in the figure. As it passes by the pore, the cytoplasmic hsp70 is released and the signal peptide is cleaved off through a signal peptidase and the protein is bound in the matrix through mitochondrial hsp70. An hsp70 is then exchanged through mitochondrial hsp60 that assists the protein to fold rightly into its last active state. Introduce of proteins into the mitochondrion needs energy from the electrochemical gradient across the inner membrane as well as ATP hydrolysis. The Protein import into the mitochondrial inner membrane and intermembrane space required two signals that are; the protein is first imported into the matrix as define above and then a second signal sequence directs the protein back into the inner membrane or across it into the intermembrane space. It import into chloroplasts follows same mechanisms to those in mitochondria but the signals used must be variant since chloroplasts and mitochondria are present together in some plant cells and since proteins become goaled to the right destination.