Lect4_ProtSort3

Lect4_ProtSort3 - x x Case 2: Signal-mediated protein...

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x x • Nuclear structure and function • Nuclear pores support bi-directional movement • Transport in (import) and out (export) of the nucleus is signal-mediated by Nuclear Localization Sequence (NLS) and Nuclear Export Sequence (NES) Case 2: Signal-mediated protein transport to: Nucleus Cell Biology – lecture 11 © Gross - all rights reserved.
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5-25: EM of a thin section of bone marrow cell • Nucleus is the predominant feature in the cell • Functions of nucleus in metabolically active cell: • Replicate DNA • Synthesize rRNA, tRNA, mRNA • What needs to be transported across the nuclear envelope? • mRNA + proteins = ribonucleic particles (mRNP) • rRNA + proteins = ribosomal subunits • tRNA particles • PROTEINS!!! It is important to segregate nucleus from cytosol to control access to DNA! Cell Biology – lecture 11 © Gross - all rights reserved.
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Nucleus surrounded by two lipid bilayers - Inner nuclear membrane defines nucleus -Outer nuclear membrane continuous with ER - Nuclear lumen continuous with ER lumen - Nuclear membranes fused at nuclear pores Structure of the nuclear envelope 3 Cell Biology – lecture 11 © Gross - all rights reserved.
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The nuclear pore • site of protein (and RNA) transport into and out of the nucleus • constitutively open, but proteins require nuclear localization signal (NLS) •Protein complex can be imported if only ONE component has NLS • bridges the inner and outer membrane • 8-fold rotational symmetry • 3 concentric rings • HUGE protein complex • 3000 to 5000 nuclear pores/cell 4
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What constitutes a NLS? NLS first discovered in mutant SV40 virus - SV40 makes a viral protein called large T-antigen - when the virus infects a cell, the wild type viral T-Antigen protein is found in the nucleus - mutant T-antigen localized to the cytoplasm - mutations for altered cellular localization all occurred within a specific seven-residue sequence rich in basic amino acids (NLS) 5 F09 D103 Cell Biology - lecture 11 © Gross- all rights reserved. Positively charged!! Uncharged; mutation decreased overall positive charge of sequence
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Fusion of a NLS to a cytoplasmic protein causes the protein to enter the cell nucleus •Fusion of the T-Antigen NLS sequence to a cytosolic protein (pyruvate kinase), results in nuclear import of the cytosolic protein •Experiment shows that the NLS sequence is necessary and sufficient to direct nuclear import a. Pyruvate kinase normally found in cytosol b. Adding the -KKKRK- sequence results in the kinase localizing to the nucleus 12-19 Lodish 6 F09 D103 Cell Biology - lecture 11 © Gross- all rights reserved.
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Signal sequence: one or two short amino acid sequences containing several positively charged lysines (K) or arginines (R) 7 The NLS can be a signal “sequence” or a signal “patch” F09 D103 Cell Biology - lecture 11 © Gross- all rights reserved.
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How are proteins imported into the nucleus?
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This note was uploaded on 12/13/2011 for the course BIOSCI d103 taught by Professor Staff during the Fall '11 term at UC Irvine.

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Lect4_ProtSort3 - x x Case 2: Signal-mediated protein...

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