Intro_import - P r o te in ta r g e tin g to o r g a n e...

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Protein targeting to organelles (MBOC 12-6) 1. Post-translational transport into nucleus through nuclear pore complex 2. cotranslation transport into the ER and then vesicle trafficking for ER, golgi, secreted, lysosomal and cell membrane proteins 3. post-translational transport into organelles such as mitochodria and peroxisomes (occasionally ER proteins) and other organelles
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Nuclear and nuclear pore structure •Nuclear structure •Double membrane with openings called nuclear pores (MBOC 12-9). •Underlying meshwork of filament protein called nuclear lamina. •Lumen between the membranes is contiguous with ER lumen.
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Nuclear and nuclear pore structure •Larger proteins are excluded in the absence of specific import process •Enables nucleus and cytosol to maintain different composition of proteins •Nuclear pore complex (MBOC 12-10) •Perforates both bilayers of the nuclear envelope •Composed of more than 50 different proteins called nucleporins •Each pore contains open aqueous channel that permits small proteins (less than 60kD) to enter freely by passive diffusion.
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Nuclear pores •Pores regulate transport of proteins and nucleic acids in and out of nucleus. •Experiment with gold particles coated with nuclear localization signals (NLS) demonstrates that transport is through the pores (MBOC 12-13). •Proteins transported in a folded state. •The pores must be able to dilate to a diameter of at least 26 nm since this is the size gold particle that can be imported if the gold particles are coated with the appropriate signal peptide.
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Transport signals and receptors •How are larger proteins and protein complexes imported / exported from the nucleus? •Signals on nuclear proteins control their entry to nucleus. •Classical nuclear localization signal (NLS) •4 to 8 amino acid stretch rich in positively charged amino acids •NLS can be situated anywhere in the protein (even split between two regions) •First identified in large T antigen protein of SV40 virus
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