The Nucleus

Nuclear Morphology

Chromosome Territories• Chromosomes occupy

discrete regions of interphase nucleus

• Chromatin – both heterochromatin and euchromatin

• Interchromasomal domain

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

Nucleolar Subdomains

• rRNA transcription at FC/DFC boundary

• Ribosomal assembly in GCs

• RNA Pol I & factors in FC

Nucleolar Organizing Regions (NORs)

• rDNA on chromosomes 13, 14, 15, 21 & 22

• Remains associated with Pol I & transcription factors throughout mitosis

Nuclear Envelope

Nuclear Lamina• Lamin B – most prevalent

• Lamin A & C – differentiation specific

Nuclear Envelope Disassembly & Reassembly During Cell Division

• Lamins A & C – dephosphorylated

• Lamin B – phosphorylated and

demethylated– Remain attached to

NE remnants

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

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

Demonstration of NLS

C

D

E

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.

Demonstration of NES

Site of injection, nuclear vs cytoplasmic, is indicated by position of arrow

Nuclear Import/Export

Cycles• Adapter = importin • Receptor = importin • Export receptor = CAS

• Ran – small GTPase• RanGAP – activates

GTPase activity• RanGEF – exchanges

GDP for GTP

Some Structures

Regulated Nuclear Localization