Student Learning Outcomes: Nucleus distinguishes Eukaryotes from Prokaryotes

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1 9 The Nucleus Student Learning Outcomes: Nucleus distinguishes Eukaryotes from Prokaryotes Explain general structures of Nuclear Envelope, Nuclear Lamina, Nuclear Pore Complex Explain movement of proteins and RNA between Nucleus and Cytoplasm Selective traffic of proteins, RNAs regulates gene expression [Describe the Internal Organization of the Nucleus] Nuclear Envelope;Traffic between Nucleus and Cytoplasm 1. Nuclear envelope: Two membranes Underlying nuclear lamina Nuclear pore complexes Outer membrane continuous with ER; membrane proteins bind cytoskeleton Inner membrane proteins bind nuclear lamina Fig. 9.1: EM of nucleus arrows indicate nuclear pores Describe the Nucleolus and rrna Processing Nuclear membrane, nuclear pores Fig. 9.1 Outer membrane is continuous with ER; Note ribosomes on ER Nuclear Envelope,Traffic between Nucleus and Cytoplasm Nuclear lamina is fibrous mesh (structural support): Fibrous proteins (lamins), and other proteins. Mutations in lamin genes cause inherited diseases Fig. 9.3: EM of nuclear lamina Each nuclear membrane is phospholipid bilayer permeable only to small nonpolar molecules. Nuclear pore complexes are sole channels for small polar molecules, ions, proteins, RNA to pass through nuclear envelope. Fig. 9.2: EM of nucleus arrows indicate nuclear pores Hutchinson-Gilford Progeria causes premature aging; Mutations in LMNA gene affect Lamin A protein 1

2 Mammals have 3 lamin genes, (A, B, and C), which code for at least 7 proteins. Two lamins form dimer, α-helical regions of 2 polypeptide chains wind around each other -> coiled coil. Lamin dimers associate to form nuclear lamina. Fig. 9.4 Nuclear pore complexes large 120 nm Complex: vertebrates, 30 different proteins (nucleoporins) Circular structures on faces of membrane; 8-fold symmetry. Lamina: loose mesh in nucleus Lamins bind: Protein emerin, lamin B receptor (LBR) (inner membrane) Chromatin. Figs. 9.5, 9,7. Nuclear Pore,Traffic between Nucleus and Cytoplasm Nuclear pore complex - 8 spokes connected to rings at nuclear and cytoplasmic surfaces. Spoke-ring assembly surrounds central channel Protein filaments extend from rings: Basketlike structure on nuclear side. Cytoplasmic filaments on cytoplasmic side Fig. 9.8 nuclear Pore complex Nuclear Pore Complex, Traffic between Nucleus and Cytoplasm Nuclear Pore Complex controls traffic between nucleus and cytoplasm: critical for physiology Passive transport: small molecules pass freely through channels Selective transport: energy-dependent for macromolecules (proteins and RNAs) Fig. 9.6 nuclear pore complex controls transport 2

3 Nuclear localization signals (NLS): Required for proteins to enter nucleus- specific aa seq Recognized by nuclear transport receptors transport of proteins through nuclear pore first identified on SV40 T antigen (viral replication protein) mutants helped figure Import of proteins to nucleus: NLS recognized by nuclear transport receptors importins Activity of nuclear transport receptors regulated by Ran, a GTPbinding protein Importins bind cargo at NLS sequence Move through pore Ran-GTP unloads, takes importin out. Some NLS are one aa seq Others bipartitate seq A, kinase with SV40 NLS; B, mutated NLS High concentration of Ran/GTP in nucleus: enzyme localization: GAP does GTP hydrolysis in cytoplasm GEF does GDP/ GTP exchange in nucleus (Fig. 9.20) Fig import of proteins Nuclear export signals (NES): Required for proteins targeted for export Signals recognized by exportins (receptors in nucleus) direct transport to cytoplasm Less well characterized than NLS Ran also required for nuclear export Ran/GTP promotes binding of exportins and their cargo proteins, Ran/GTP dissociates complexes between importins and cargos (see Fig. 9.10) Fig export of proteins Many importins and exportins are family of nuclear transport receptors - karyopherins. 3

4 Regulation of Protein transport is another point at which nuclear protein activity can be controlled: Regulation of import, export of transcription factors: Inhibitors block import (IkB and NF-kB) phosphorylation can block import (de-po 4 releases) Fig regulated import Most RNAs are exported from nucleus to cytoplasm to function in protein synthesis: Active, energy-dependent process requires transport receptors Transported as ribonucleoprotein complexes (RNPs). rrnas associate with ribosomal proteins, specific RNA processing proteins in nucleolus (Fig. 9.31). mrnas associate with 20 proteins during processing, transport Fig EM of RNP transport : insect salivary gland; RNA unfolds Fig 9.15 Transport of snrnas between nucleus and cytoplasm Many small RNAs (snrnas, snornas) function in nucleus. snrnas are transported to cytoplasm by exportin (Crm1) associate with proteins to form snrnps and return to nucleus; snrnps function in splicing Internal Organization of the Nucleus 2. Internal structure of nucleus: organized, localized In animal cells, lamins where chromatin attachmes, organize other proteins into functional nuclear bodies Heterochromatin highly condensed, transcriptionally inactive; Euchromatin decondensed, all over Chromosomes organized in territories: Actively transcribed genes at periphery Fig RNA Fig arrow = nucleolus; arrowheads = heterochromatin Fig mammalian nucleus: DNA probes to chrom 4 4

5 Internal Organization of the Nucleus Nuclear processes appear localized (sequestered) to distinct subnuclear regions: DNA replication: Mammalian cells: clustered sites labeling newly synthesized DNA with bromodeoxyuridine (BrdU in place of T) Immunofluorescence (Ab to BrdU): newly replicated DNA in discrete clusters Internal Organization of the Nucleus Nuclear processes appear localized (sequestered) to distinct subnuclear regions nuclear speckles: mrna splicing machinery Detect with immunofluorescent staining - antibodies against snrnps and splicing factors. PML bodies have transcription factors, chromatin-modifying proteins; identified from protein in promyelocytic leukemia Fig. 21 A: early replication B, late replication Fig Speckles Fig PML bodies The Nucleolus and rrna Processing *3. Nucleolus: Site of rrna transcription, processing, some aspects of ribosome assembly. Actively growing mammalian cells have 5 to 10 x 10 6 ribosomes, must be synthesized each time cell divides. Nucleolus is not surrounded by a membrane Multiple copies of rrna genes (200 human) In oocytes, rrna genes amplified, synthesis for early development. rrna genes amplified 2000-fold in Xenopus oocytes, thousands of nucleoli, ribosomes per oocyte Fig Xenopus oocyte rrna genes The Nucleolus and rrna Processing Fig Nucleolar organizing regions: After each cell division, nucleoli reform, associated to genes for 5.8S, 18S, and 28S rrna genes Each nucleolar organizing region has tandemly repeated rrna genes separated by spacer DNA 5.8S, 18S, and 28S rrnas are transcribed as single unit in nucleolus by RNA pol I 45S ribosomal precursor RNA Fig

6 Fig 9.29 Processing of pre-rrna Primary transcript of rrna genes is large 45S pre-rrna pre-rrna processed via series of cleavages, and some base modifications, including methylations snornps (snornas with proteins) assemble on pre-rrna as processing complexes (like spliceosomes on pre-mrna) Fig 9.31 Ribosome assembly Formation of ribosomes requires assembly of pre-rrna with ribosomal proteins and 5S rrna, then export of subunits pol II made the mrna for ribosomal proteins. Fig ETS, external transcribed ITS, internal transcribed Fig Review questions: 1. Eukaryote nuclear membranes separate transcription from translation. What regulatory mechanisms unique to eukaryotes achieve this regulation? 3. If you inject a frog egg with two globular proteins, one 15 kd and the other 100 kd, both of which lack NLS, will either protein enter the nucleus? 4. What determines the directionality of nuclear import? 5. Describe how the activity of a transcription factor can be regulated by nuclear import. * Consider the effect of mutations at gene level that inactivate NLS, NES, prevent phosphorylation of key sites, or prevent binding inhibitors on function 6