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17. Nucleo cytoplasmic transport.pdf

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McGill University
Biology (Sci)
BIOL 200
Richard Roy

Naveen Sooknanan McGill Fall 2011 Nucleo-cytoplasmic Transport: So far, all the mechanisms we have seen on DNA and RNA have taken place in the nucleus. Protein synthesis, however, takes place in the cytoplasm. So, these final mRNAs must somehow be transferred from the nucleus to the cytoplasm and then associate with a bunch of new proteins associated with translation. Nuclear pores are complexes which resemble baskets and are located all over the nuclear membrane, which has a bilayer membrane  They make the nucleus look like a golf ball structure because of the perforations  Nuclear pore complexes are the sites of nucleo-cytoplasmic transport o This process is a two way road; some proteins synthesized in the cytoplasm need to be brought into the nucleus, while RNAs such as mRNA need to be brought out of the nucleus  The nuclear pore complex is a highly organized structure made of many protein subunits o They contain nucleoporins with Fg repeat proteins, which are zones of increased hydrophobicity  Much of the transport through the nuclear membrane is mediated by these Fg repeats  The NPC itself is huge, over 125 megadaltons in size (30 times bigger than a ribosome) o It is composed of many proteins; 50 in yeast and 100 in humans  Molecules up to 60 kDa can diffuse freely through the NPC, but larger complexes require multimolecular transporters (RNPs) to be actively transported It is by the interaction between various transporter proteins and the hydrophobic regions of the Fg repeats within the NPC that molecules get in and out of the nucleus  Nuclear proteins, such as splicing factors, transcription factors, etc. must be brought back into the nucleus through the NPC  All of these proteins contain a nuclear localization signal (NLS) which allows them to be recognised by specific transporter protein o NLS’s function has been seen by adding an artificially labelled NLS onto pyruvate kinase, which is normally a cytoplasmic protein  All the proteins immediately go to the nucleus, as can be seen by the concentrated fluorescence o NLS proteins are rich in lyseines and arginines, and with practice, one can almost guess which proteins belong in the nucleus There are two proteins which are heavily involved in the nuclear import mechanism: Ran and Importin α and β  The first step in nuclear importing is the importin α binding to the NLS of the cargo protein (the protein to be imported)  Importin β binds to the Fg nucleoporins and the interaction of importins β and α allow the cargo protein to be imported into the nucleus 1Cytoplasm Nude plasm GAPS Cytoplasm Nude plasm GAPSNaveen Sooknanan McGill Fall 2011 o This process is driven by a concentration gradient of cargo protein across the nuclear membrane  Cargo protein is in higher concentration in the cytoplasm than the nucleus, causing a net flow into the nucleus  Ran is a monomeric GTP binding protein which has a different conformation whether it is bound to GTP or GDP o GTP bound Ran is in its active form, and hydrolysis of GTP causes GDP bound Ran to enter an inactive state through a conformational change o GTP binding proteins (there are many of them) are triggers, or switches, in the cell which can be used to drive many different reactions The mechanism for nuclear import is Ran dependant. The whole process is dependent on the placement of GEF and GAP, two proteins responsible for the activation and inactivation of Ran.  Importin forms a complex with the cargo. This complex is brought though the NPC into the nucleus o This is driven by a concentration gradient where importin/cargo complex is high in the cytoplasm and low in the nucleus  Ran-GTP then binds to importin, which lets go of the cargo, which is free to perform its nuclear function o Ran GTP is activated in the nucleus by GEF by the phosphorylation of GDP o Binding of Ran-GTP to importin
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