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Lecture 18

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Biology (Sci)
BIOL 201
Greg Brown

Lecture 18 Small subunit of the rubisco serves the purpose of a regulator, encoded in the nucleus, while the large subunit is the catalytic and encoded by the chloroplast. The small subunit has a precursor with N terminus transit peptides that allows for the entry into the stroma, powered by Hsc70 ATP hydrolysis, not as a result of the H gradient. The transit peptide is removed, and do not target the proteins to the mitochondria, and vice-versa. The small subunit is associated with cpHsc70, the large subunit with Hsc60 before assembly. 4 pathways exist for proteins to enter the stroma and several of them have no homologous proteins to bacterium transferring proteins. Plastocyanin, for example. When the first segment is removed, a signal is exposed, which then associates with SRP, allowing for the transfer of the protein into the stroma. Nuclear Pore Transport: Unlike the ER, the chloroplast and the mitochondria, the proteins enter in a folded state. These proteins must enter via the nuclear pore basket complexes. 3 types of nuclearporins, membrane, structural and FG proteins. These FG are made of phenylalanine and glycine repeats, very disorganized in shape, very important for transport mediation. 16 complexes set up the Y complex, made of structural nuclearporins, the core of the complex. The FG proteins line the pore made from the Y complex. Cargo proteins, can be either small or large, small ones could just pass through without Nuclear Localization Signals. Larger proteins that are greater than 40kDa require NLS. Ran and Importin are the two transporters for large proteins. These transporters have NLS, which is on the C-terminus, which is also not removed, sometimes even found in the protein itself, not on the ends. The NLS protein (importins) can bind to the FG proteins which lines the pore itself. The FG forms a gel like matrix that controls entry and exit of the nucleus. The protein is then diffused down a concentration gradient into the nucleus. Once inside, Ran GDP is phosphorylated into Ran GTP, which then binds to Importin, the importin then loses affinity to the cargo, drops the cargo off. The Ran GTP + importin, which is then able to move out of the nucleus into the cytoplasm. Once outside, a GAP then hydrolyzes Ran GTP into Ran GDP. Ran GDP and Importin break apart, now importin is back outside. Protein movement through Golgi apparatus: Proteins move into the cis-Golgi network and the compa
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