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RNA editingis an uncommon process in most eukaryotic cells. Deamination reactionsconvertA → I & C → U by removing amines. In mammalians, a deaminating enzyme in the intestingchanges a single nucleotide and this reduces the apoB protein by half the size by forming a stopcodon.
Lecture 24: Nucleo-Cytoplasmic TransportTranslation occurs in the cytoplasm and it is important to get the mRNA out of the nucleus.How? By using nuclear pore complexes which are highly ordered structures. They act asgatekeepers since large molecules (ribosomes, spliceosomes) can’t go through. The nuclearpore complex (NPC)is a huge structure (30x bigger than a ribosome).Transport through the NPC depends on reversible hydrophobic interactions. Transporters connectwith the hydrophobic regions (FG domains)in order to go through. Nuclear proteins synthesizedin the cytoplasm must also go through the NPC. These proteins contain a nuclear localizationsignal (NLS)which can be about 6 amino acids. A protein required for nuclear import is Ran, a G protein that interacts with the importins(nuclear transport receptors). These proteins bind to NLS domains present on cargo proteins tofacilitate transport through the pore by association with FG-nucleoporins. There is little Ran-GDP in the nucleus so the concentration gradient makes the import complex (Ran-GDP + cargoprotein + importin) enter the NPC. Exportin 1is a Ran-dependent nuclear export protein. It associates with Ran-GTP and the cargoprotein which has a different signal called NES. Some RNAs are exported through associatingwith Ran. Exportin tbinds to tRNAs and Ran-GTP. Other RNAs include ribosomal subunits andhnRNP proteins (HIV). Most mRNAs are exported in a Ran-independent process using a mRNAexporter.The mRNA exporter consists of 2 subunits: Nxf1 & Nxt1. Together, they bind RNA via SRproteins and form a domain that interacts with FG repeats. The mRNA undergoes mRNPremodeling. A RNA helicase rips off SR proteins, the exporter and all secondary structures andthey are replaced by cytoplasmic analogues. PABPII, the CAP binding protein, Nxf1/Nxt1, SRproteins and splicing factors go back in the nucleus. PABPII is replaced by PABPI.The 5’ end of the mRNA is exported first as ribosomes associate with it while it is still beingexported and the RNA helicase leaves the mRNA naked and ready for translation. Only fullyspliced mature mRNAs get exported. Anything associated with pre-mRNAs is blocked, possiblyby a protein bound to a nucleoporin.
Lectures 25 & 26: Protein synthesisRNAs play roles in all aspects of translation: rRNAbuilds the ribosomal machine, mRNAhasthe codons and tRNAbrings the amino acids. Also, several codons code for the same amino acidmeaning that the genetic code is degenerate.Pre-tRNAs undergo critical processing: 1)The 5’ end sequence is removed. 2)A short intron isremoved.