Nuclear Structure and Protein Import

Why the need for a Nucleus:● Prokaryotes have existed for millennia and are successful, despite having no nucleus● Eukaryotes are also successful and tend to be more complex, multicellular organisms
The Interphase Nucleus is a Highly Organized Place:Chromosome territories:● interphase chromosomes are spatially organized● chromosomes are generally not all tangled together
Components of a NucleusNuclear Envelope:● the outer nuclear membrane is continuous with the rough ER and has ribosomes attached● nuclear lamina aids in disassembly and re-assembly of the nuclear envelope during cell division● phosphorylation is a mechanism that cells use to regulate function          •phosphorylation = covalent attachment of a phosphate group
Nucleolus:● the nucleolus has no membrane● specialized chromosomal region of the nucleus ● function:          • site of rRNA synthesis and assembly of ribosomal subunits made from rRNA and ribosomal proteins● chromosomal regions that form the nucleolus have 100-200 copies of rRNA ● 5 Pairs of chromosomes in the human genome that contain rRNA encoding regions
Ribosomes are made of rRNA and protein:● rRNA is transcribed in the nucleolus● rRNA and ribosomal proteins are assembled in the nucleolus ● ribosomal subunits (small and large) come together during translation in the cytoplasm
Heterochromatin and Euchromatin ● Describe the degree of DNA packing along a single interphase chromosome
HETEROCHROMATIN:-Densely Staining Regions-about 10% of interphase chromosome-concentrated around centromeres, and telomeres of chromosomes -not many active genes included in heterochromatin
EUCHROMATIN-Lightly Staining Regions-contains less condensed DNA-genes within this region are actively being transcribed
Nuclear Pore:● large multi-protein subunit complex that acts as a nuclear gate ● diameter ~ 90-120nm ● predicted opening ~9nm in diameter, but active transport can move molecules up to ~40nm

Exported Out:

● Assembled ribosomal subunits● mRNA● tRNA

Two Requirements for targeting Proteins to Specific Locations

1. A specific signal sequence in the transported protein2. A specific protein receptor that recognizes that signal sequence
For proteins that enter/exit the nucleus:● The transported protein contains a targeting signal in its primary amino acid sequence ● import: NLS = Nuclear Localization Signal● export: NES = Nuclear Export Signal
Nuclear Transport receptor: Bind to nuclear signal sequence and can dock onto proteins of the nuclear pore● NIR = Nuclear Import Receptors● NER = Nuclear Export Receptors
NUCLEAR IMPORT ANIMATION = https://youtube.com/watch?v=UyhqLpjicZg
Active transport requires energy - the take away message is that the hydrolysis of GTP is required, thus providing the energy to allow for nuclear import
Targeting Signals:● targeting signals are encoded within proteins ● targeting signals direct the protein to a specific organelle ● targeting signal must be present for protein to leave the cytosol● proteins with no targeting signals remain in the cytosol
will include NLS case study! LINK: