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The Nucleus

This article delves into the complex structure and function of the nucleus, highlighting the organization of chromosome territories during interphase. It discusses the roles of chromatin types, RNA synthesis sites, speckles, and the dynamics of nucleolar subdomains in rRNA transcription and ribosomal assembly. Additionally, it examines nuclear envelope components, including lamins and nuclear pore complexes, and their involvement in nuclear trafficking. The significance of nuclear localization signals (NLS), export signals (NES), and retention signals (NRS) in protein import and export is also explored.

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The Nucleus

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  1. The Nucleus

  2. Nuclear Morphology

  3. Chromosome Territories • Chromosomes occupy discrete regions of interphase nucleus • Chromatin – both heterochromatin and euchromatin • Interchromasomal domain

  4. Sites of RNA Synthesis and Processing • Perichromatin fibrils – >10000 sites of Pol II transcription & RNA processing • Speckles & Cajal bodies • sites of storage for RNA splicing components

  5. Nucleolar Subdomains • rRNA transcription at FC/DFC boundary • Ribosomal assembly in GCs • RNA Pol I & factors in FC

  6. Nucleolar Organizing Regions (NORs) • rDNA on chromosomes 13, 14, 15, 21 & 22 • Remains associated with Pol I & transcription factors throughout mitosis

  7. Nuclear Envelope

  8. Nuclear Lamina • Lamin B – most prevalent • Lamin A & C – differentiation specific

  9. Nuclear Envelope Disassembly & Reassembly During Cell Division • Lamins A & C • dephosphorylated • Lamin B • phosphorylated and demethylated • Remain attached to NE remnants

  10. Nuclear Pore Complexes • ~ 3000-5000 pores/nucleus • 130 nm in diameter • 55 nm central channel • Mammalian pores composed of >100 proteins • Nucleoporins major class of pore proteins

  11. Nuclear/Cytoplasmic Trafficking • Nuclear Localization Signal – NLS • Many defined a. a. sequences • Basic NLS – very common • Stretch of basic a.a. residues • SV-40 T Ag – PKKKRKV • Direct import of protein into nucleus • Nuclear Export Signal – NES • Direct export of nuclear proteins • Critical for recycling of import factors • Nuclear Retention Signal – NRS • Critical for retention of various RNA processing factors

  12. Demonstration of NLS C E D F Figure 16-13. A NLS ICAD causes specific import into the nucleus of an ICAD-GFP fusion protein. Panels A, C & E show cells transfected with ICAD-GFP without a NLS, panels B, C & F are cells transfected with ICAD-GFP with a NLS. A & B- Composite fluorescence image of GFP and DAPI. C & D- GFP fluorescence showing cytoplasmic distribution of ICAD-GFP lacking a NLS (C). E & F- Nuclei stained with DAPI.

  13. Demonstration of NES Site of injection, nuclear vs cytoplasmic, is indicated by position of arrow

  14. Nuclear Import/Export Cycles • Adapter = importin  • Receptor = importin  • Export receptor = CAS • Ran – small GTPase • RanGAP – activates GTPase activity • RanGEF – exchanges GDP for GTP

  15. Some Structures

  16. Regulated Nuclear Localization

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