Transcript ppt

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]
• Describe the Nucleolus and rRNA Processing
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
Nuclear membrane, nuclear pores
Fig. 9.1 Outer
membrane is
continuous with ER;
Note ribosomes on ER
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
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
Hutchinson-Gilford Progeria
causes premature aging;
Mutations in LMNA gene
affect Lamin A protein
Nuclear Envelope, Traffic between Nucleus and Cytoplasm
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 Envelope, Traffic between Nucleus and Cytoplasm
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
Nuclear Envelope, Traffic between Nucleus and Cytoplasm
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
Some NLS are one aa seq
Others bipartitate seq
A, kinase with SV40 NLS; B, mutated NLS
Nuclear Envelope, Traffic between Nucleus and Cytoplasm
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.
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. 9.11 import of proteins
Nuclear Envelope, Traffic between Nucleus and Cytoplasm
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. 9.12 export
of proteins
Many importins and exportins are family of nuclear
transport receptors - karyopherins.
Nuclear Envelope, Traffic between Nucleus and Cytoplasm
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-PO4 releases)
Fig. 9.13
regulated import
Nuclear Envelope, Traffic between Nucleus and Cytoplasm
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. 9.14 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
Fig. 9.15 RNA
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. 9.16 arrow = nucleolus;
arrowheads = heterochromatin
Fig. 9.19 mammalian nucleus:
DNA probes to chrom 4
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
Fig. 21 –
A: early replication
B, late replication
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. 9.22
Speckles
Fig. 9.23 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 106
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,
→1012 ribosomes per oocyte
Fig. 9.26 Xenopus
oocyte rRNA genes
The Nucleolus and rRNA Processing
Fig. 9.28
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. 9.25
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. 9.29
ETS, external transcribed
ITS, internal transcribed
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. 9.31
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