L38- Protein Trafficking and Nuclear Transport

• What do all eukaryotic cells have in common?

  A basic set of membrane-enclosed organelles
• How do eukaryotic cells differ visually?


  They can look extremely different
• What is the composition of the cytoplasm?

  Cytosol plus organelles
• What percentage of cell volume does the cytosol occupy?

  50%
• What is the primary function of the nucleus?

  Site of DNA and RNA synthesis
• What organelle is responsible for generating ATP?

  Mitochondria
• What is the role of the Golgi apparatus?

  Sorting and modifying proteins and lipids
• What are lysosomes known as?

  “Suicide” bags for digestion
• What is the function of peroxisomes?

  Sites for oxidative reactions
• What is the primary function of plant vacuoles?

  Turgor or protein storage/degradation
• What is meant by compartmentation in eukaryotic cells?

  Presence of membrane-enclosed organelles
• Where does protein synthesis begin?

  In the cytosol
• What governs the transport and localization of proteins?

  Intrinsic signals within the proteins
• What is gated transport?

  Import and export into the nucleus
• What are sorting signals in proteins?

  Signals that direct proteins to their destinations
• What can sorting signals be composed of?

  Short peptides, 3D domains, or modifications
• What happens to sorting signals after use?

  They can be removed or retained
• How are sorting signals recognized?

  By specific receptors within the cell
• What can result from mis-sorting of proteins?

  Several diseases due to traffic jams
• What do nuclear pore complexes (NPCs) facilitate?

  Movement of macromolecules between cytoplasm and nucleus
• How many nuclear pore complexes are in a mammalian nuclear envelope?

  3000-4000 NPCs
• What is the size comparison between ribosomes and NPCs?

  NPCs are 30 times bigger than ribosomes
• What is the diffusion rate of small molecules?

  They diffuse extremely fast
• What is the role of nuclear localization signals (NLS)?

  To facilitate active transport into the nucleus
• What do importins do?

  They are NLS receptors for nuclear import
• How do FG repeats assist import receptors?

  They help recruit receptors and bind cargo
• What drives the net import of cargo in nuclear transport?

  Conformational states of the GTPase Ran
• What is the role of GAP and GEF in regulating Ran-GTPase?

  They switch Ran on or off
• What happens during nuclear export?

  Exportins bind NES and FG-nucleoporins
• What is the significance of Nup98 in NPCs?

  It activates genes related to leukemia
• What is the summary of protein trafficking in eukaryotic cells?

  • Proteins need signals for correct intracellular destination
  • Sorting signals are within the protein
  • Cytoplasm-nucleoplasm traffic occurs through NPCs
  • Uses importins for nuclear import and exportins for nuclear export
  • Directionality is provided by Ran GTPase switch
  • Asymmetric distribution of Ran GTP and Ran GDP is crucial
• Basic set of membrane enclosed organelles:

  Nucleus - site of DNA + RNA synthesis

  Cytoplasm - cytosol + organelles

  Cytosol - 50% cell volume, site of protein synthesis and many metabolic pathways

  ER

  Golgi apparatus

  Lysosomes

  Mitochondria/chloroplasts

  Plant vacuoles
• How do proteins know where to go?

  They use SORTING SIGNALS

  (... Carry on notes later)
• Nuclear Localisation Signals (NLS) are recognised by a family of cytosolic nuclear import receptors (importins/karyopherins) each member belong responsible for asset of cargo molecules (or sometimes a cargo adaptor)
• Why is gated transport (active nuclear transport) considered 'active'?

  Moving certain proteins inside the nucleus create different protein pools in the nucleoplasm compared to the cytoplasm. This type of nuclear transport creates order in the cell (ORDER = energy required to maintain it). The energy for this type of transport comes from GTP via the GTPase-Ran
• What does GTPase do?

  It binds to a 'switch' protein called Ran.

  It then hydrolyses GTP to GDP.

  The conformation is different depending if bound to GTP or GDP

  Different conformation = different activity