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notesxm2 - Chapter 6 The nuclear envelope has a complex...

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Chapter 6 The nuclear envelope has a complex structure consisting of two nuclear membranes, - to act as a barrier that separates the contents of the nucleus from the cytoplasm an underlying nuclear lamina, nuclear pore complexes - fibrous meshwork - structural support to the nucleus Lamins are 60 to 80 kilodalton fibrous proteins that make up the nuclear lamina. Nuclear PORE complexes only channels thru which small polar molecules, ions, & macromolecules can travel bw the nucleus and the cytoplasm. Depending on their size, they travel by two different mechanisms. - Passive diffusion – small molecules - Energy dependent transport – protein, RNAs Nuclear localization signals (NLS) - specific aa sq that are recognized by transport receptors and direct the transport of proteins thru nuclear pore complex. Nuclear transport receptors are proteins that recognize NLS and mediate transport across the nuclear envelope. Karyopherin is a nuclear transport receptor, - Importins transport macromolecules to the nucleus from the cytoplasm. - Exportins transport macromolecules from the nucleus to the cytoplasm. Ran is one of several types of small GTP- binding proteins that regulate movement of macromolecules through the nuclear pore. Ran controls the activity of the nuclear transport receptor (karyopherin). Ran/GTP promotes - importin/cargo protein dissociation - exportin/cargo protein association Protein import through the nuclear pore complex begins when - specific importin binds to the NLS of a cargo protein in the cytoplasm. Nuclear export signals (NES) are specific aa sq that target proteins for export from the nucleus Transport of RNAs RNAs are transported across the nuclear envelope as ribonucleoprotein complexes, or RNPs Most tRNAs and rRNAs are transported by karyopherin in a Ran/GTP-dependent manner. mRNAs are transported by “mRNA exporter” independent of Ran Nucleolus - the site of rRNA transcription and processing the most prominent(most important) nuclear body the site of rRNA transcription and processing the site of ribosome assembly. The final stages of ribosome maturation follow the export of pre-ribosomal particles to the cytoplasm, forming the active 40S and 60S subunits of eukaryotic ribosomes. The endoplasmic reticulum, or ER , is a network of membrane-enclosed tubules and sacs that extends from the nuclear membrane throughout the cytoplasm. The rough ER is covered by ribosomes on its outer surface and is involved in protein synthesis . Secretive proteins - Proteins synthesized from free ribosomes are normally retained in cells. The smooth ER is involved in lipid metabolism and is not associated with ribosomes .
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