BCH3021 Lecture Notes - Lecture 12: Retinoic Acid Receptor, Nuclear Membrane, H2Afy
Lecture 12 – Nuclear Functions cont.
Subnuclear Compartments
1. Nucleolus – ribosome assembly
2. Specialised subdomains (no membrane enclosed structures – diffusion)
a. Sc35 Domains (speckles)
• Splicing factor compartments (pink)
• 20-40/nucleus
• Contain splicesome assembly factor SC35
• Dynamic structures near highly expressed/complex genes
enabling rapid use of stored factors for splicing
b. PML Nuclear Bodies (PML NBs)
• Spherical structures
• 10-20/nucleus
• Stress response (protective)
• Contain PML protein (promyelocytic leukemia) and 40
different proteins
• In acute PML, PML is fused to retinoic acid receptor (gene
translocation) → breakdown of PML NBs
• Nuclear depots to which proteins are recruited during
stress/virus infection (role in gene expression)
• Viral proteins/stress disrupt PML NBs
c. Cajal (coiled) bodies (CBs) – Nucleolar Accessory Bodies
• 0.1-1.5 u spherical structures
• 1-5/nucleus
• Contain coilin protein, as well as numerous snRNAs/nucleolar
derived proteins (e.g. ELL)
o Function in RNA processing, snRNP recycling
• Loss of CPs linked to leukemia
• Translocation results in formation of chimeric MILL-ELL
fusion protein → CB disruption
d. Barr Body
• Only one in nucleus only in females
• Inactive chromatin/gene dosage
• Inactivation of 1 of the 2X chromosomes in mammalian
females occurs in embryogenesis to ensure that all cells (male
and female) synthesise equal amounts of X-encoded gene
productions
• Chromatin: more condensed,
hypermethylated/hypoacetylated core histones, contains
histone macroH2A variant
• X inactivation is effected by XIST (X-inactivation
specific transcript gene) which produces a large poly A-
RNA that coats inactive X
Nuclear Envelope Breakdown During Mitosis
• Breakdown and reformation of nuclear envelope during mitosis
o Driven by nuclear lamin (skeletal structure of nucleus)
phosphorylation and takes a bit with nucleus envelope
find more resources at oneclass.com
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o To form nuclear envelope: dephosphorylate lamin → form mini
nucleus and nuclear envelope
▪ Need to make sure enzymes or anything else doesn’t sneak in
• Interphase nucleus → phosphorylation of lamins → prophase →
dephosphorylation of lamins → early telophase → fusion of nuclear envelope
fragments → late telophase → fusion of enveloped chromosomes →
interphase nucleus
Getting Molecules into the Nucleus
• Directionality of transport through NPC
o Things that enter:
▪ Ribosomal proteins
• Molecular sieve function of the NPC
o Hole in middle for things to diffuse
through
o MW cuttoff of 45 kDa for a globular
protein
o Molecules < 45 kDa can diffuse freely
between nucleus and cytoplasm
o Molecules > 45 kDa require a specific
targeting signal (nuclear localisation
sequence – NLS) to traverse the NPC
• Nuclear localization sequences (NLSs)
o Short modular sequences of mostly
basic amino acids
o 2 main types:
▪ Monopartite NLCs: NLS of simian virus SV40 large tumour
antigen
• Single amino acid substitutions abolish NLS function
→ protein is cytoplasmic
▪ Bipartite NLS: comprise 2 clusters of basic residues separated
by spacer of 10-12 amino acids
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Interphase nucleus phosphorylation of lamins prophase dephosphorylation of lamins early telophase fusion of nuclear envelope fragments late telophase fusion of enveloped chromosomes interphase nucleus. Protein is cytoplasmic: bipartite nls: comprise 2 clusters of basic residues separated by spacer of 10-12 amino acids, mutation is either basic cluster of bipartite nls impair. Impa/b-mediated nuclear import pathway: docking, requires nls receptor (heterodimer of 2 proteins the importin) for nuclear import and subunits. Rangdp: docking of rangdp at npc mediated by ntf2, ntf2: nuclear transport factor-2, ran: guanine nucleotide binding protein, conversion of rangdp rangtp in nucleus by rcc1, rcc1: guanine nucleotide exchange factor (gef) for. Ran: nucleoplasmic release effected by rangtp binding to imp, nls and imp knock off, release of imp from imp is also assisted by binding to imp of particular nups (nup50) and. Cas/rangtp: cas: nuclear exporter for imp , help push away from , when ran is binding to interaction is weaker.
