Southern Methodist University - BIOL 3350chapter 15 cell bio

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Protein translocators: - imported mitochondria and chloroplast proteins o most have a signal sequence at the N-terminal that allows them to enter a specific organelle. o Proteins destined for eith... er organelle are translocated simultaneously across both inner and outer membranes as specializes sites where the two membranes contact each other. o Each protein is unfolded as it is transported and its signal sequence is removed after translocation is complete. o Chaperone proteins- also assist in pulling the protein across the two membranes and fold it back once it is inside. o The insertion of the transmembrane proteins into the inner membrane is guided by signal sequences in the protein that start and stop the transfer across the membrane like the insertion for proteins in the ER membrane. Mitochondrial protein translocator process: 1. the mitochondrial signal sequence on a mitochondrial protein is recognized by a receptor on the outer mito membrane. 2. This receptor is associated with a protein translocator a. This complex diffuses laterally in the outer membrane until it encounters the inner membrane translocator. 3. The two translocators then transport the protein across both membranes unfolding the protein in the process. 4. The signal sequence is finally cleaved off by signal peptidase in the mitochondrial matrix. Same process for chloroplasts and the chaperone proteins pull the protein across membranes and help refold. Proteins enter peroxisomes from both the cytosol and the endoplasmic reticulum: Peroxisomes: these organelles are present in all eukaryotic cells, and break down a variety of molecules such as alcohol, fatty acids, and toxins.  they acquire the bulk of their proteins from the cytosol from selective transport.  A short sequence of 3 amino acids serves as an import signal for many peroxisomal proteins.  This signal is recognized by receptor proteins in the cytosol.  They enter via protein translocators called peroxins. o But the do not need to unfold to enter  Most peroxisomal proteins come from the cytosol, some come via vesicles that bud from the ER. Proteins enter the endoplasmic reticulum while being synthesized. - the endoplasmic reticulum- serves as an entry point for proteins destined for other organelles as well as the er itself. 9 [Show More]

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