28 - Introduction To Protein Trafficking Flashcards
All organelles in cell and function
Nucleus - site of DNA and RNA synthesis
Cytoplasm - cytosol + organelles
Cytosol - 50% cell volume, site of protein synthesis and many metabolic pathways
Organelles in the cytoplasm:
ER - 50-60% cell membrane, start point of secretory pathway
Golgi apparatus - 10% cell membrane - important for sorting and modifying proteins and lipids passing through it
Peroxisomes - 1% cell volume - multiple sites for oxidative reactions
Lysosomes - 1% cell volume, multiple “suicide” bags for digestion of materials
Mitochondria/chloroplast (plants) - 25% cell volume, generate ATP
Plant:
Plant vacuoles - 90% cell volume, for turgoror protein storage/degradation
How do proteins, not residing in cytosol, know which compartment it needs to go to?
- proteins not residing in cytosol need sorting signals to reach their correct destination
- these signals are part of the protein
- signals can be:
- short peptides are N- or C-termini - removed after use or kept if needed again
- 3 dimensional domains (2/3 structure) - e.g. for transport to lysosomes
- other molecules attached to the protein - e.g. sugars, and lipids
What happens to sorting signals on proteins
- recognised by specific receptors
- in turn triggers transfer of client protein to correct destination
- every organelle has different receptors and sorting processes
- if any of this process goes wrong, cell can have problems - e.g. diseases from missorting of proteins (traffic jams)
3 modes of protein transport
- gated transport
- transmembrane transport
- vesicular transport
Example of gated transport
Import into and export out of the nucleus (nuclear import)
Examples of transmembrane transport
Protein import into ER and mitochondria uses transient translocation channels in correct membrane
Example of vesicular transport
Secretion along organelles of the secretory pathway - targeting to lysosomes
Gated transport into the nucleus
- Nucleus contains 3000-4000 nuclear pores
- proteins and other macromolecules (e.g. ribosomes) move between cytoplasm and nucleus
- via large aqueous nuclear pore complexes
Size of molecules that move into nucleus from cytoplasm, and active transport info
- small molecule (5kDa or less) can rapidly diffuse between cytoplasm and nucleoplasm
- proteins of 20-40,000 Da diffuse more slowly
- proteins larger than 40kDa cannot enter, and DNA/ribosomes cannot exit, unless they carry nuclear localisation or export signals
- 22% of human proteins are nuclear and must be imported from cytosol
- assembled ribosomes have to be esported
Why only small proteins pass through nuclear pore by diffusion, and function of diffusion barrier in nuclear pores
- proteins with diameter of 9nm can pass via diffusion
- 26nm diameter proteins can pass through by active transport
- caused by diffusion barrier in nuclear pores
Diffusion barrier in nuclear pore info
- caused by unstructured regions of NPC proteins
- form a tangled network
- blocks passive diffusion of large molecules
How big are nuclear pore complexes (NPCs)
125x10^6 Da
- made of many copies of ~30 different nucleoporins
- FG-nucleoporins line the channel, nuclear basket and cystosolic fibrils
Nuclear localisation signals (NLS) info
- NLS are rich in lysine and proline
- they can be in any position of the passenger (cargo) protein
- so long they are exposed to the surface of the protein
- import ins are the receptors for the NLS
What are receptors for the NLS
Importins
- a family of cystosolic nuclear import receptors
- each importin responsible for a set of cargo molecules (or cargo adapter)
Nuclear import via importins
