Transport Flashcards
What is GPCR?
It is the largest class of human cell receptors. Smell, taste, vision, hormone detectors. It has 7 transmembrane domains.
What is a G protein
Peripheral membrane protein
What is the structure of a G protein
3 protein. Alpha, beta and gamma proteins.
Alpha and gamma proteins have tails that bind to the bilayer.
Alpha subunit= is a GTPase so can bind to GDP and GTP.
What is the G protein activation cycle?
1.Ligand binds to GPCR causing a conformational change.
2. This recruits a G protein. The G alpha protein gets the GDP substituted to GTP.
3. This activates G protein and causes signalling cascade.
4. RGS changes GTP to GDP inactivating G alpha.
5. GRK (receptor kinase) phosphorylates GPCR and arrestin inactivate GPCR.
What does PKA do?
It is a kinase so it can modulate transcription (gene expression)
Phosphorylates many targets.
What is the structure of PKA and what does it require?
It has 2 regulator subunits and 2 catalytic subunits. Regulator subunit has cAMP receptors. When bound, the regulator and catayltic subunits separate.
What does Gq do?
When ligand binds, GPCR undergoes conformational change, the Gq protein is activated , PLCb (phospholipase b), secondary messengers diacylglycerol and inositol 1,4,5 triphosphate bind to produce PL4,5-bisphosphate.
Diacylglycerol activates protein kinase C= Phosphorylation enzyme.
What does IP3 do?
its soluble
it binds to calcium channels in ER, Ca2+ released and enters the cytosol. Ca2+ is an important secondary messenger. It activates protein kinase C.
What are the cellular effects of calcium?
Activates calmodulin which binds to calcium ion.
What does RTK stand for?
Receptor tyrosine kinase.
How does RTK binding work?
When a ligand binds to the kinase, they dimerise and phosphorylate each other.
What is the MAPK pathway?
Where dimerisation takes place and the RTK dimerises. Then the Ras-activating protein binds to it and the GDP is phosphorylated to GTP and this activates ras protein which allows signal to be transmitted. The activated ras protein allows for MAP kinase kinase kinase to be phosphorylates MAP kinase kinase etc…. It is used for changes in protein activity and gene expression.
How receptor signalling is good?
- endosomes: low pH so ligand dissociates.
- lysosome: entire receptor ligand complex is broken down in lysosome.
- receptor inactivation: phosphatases.
- Inactivation of signalling protein
- production of inhibitory proteins and transcription regulators.
How are proteins moved from one place to another?
They have amino acid sequences on them. Nuclear import receptors are soluble cystolic proteins and recognise signal sequences and so deliver proteins to the nuclear pore.
What is transmembrane transport?
Uses membrane bound translocators.
for:
- proteins made in cytosol for mitochondria or ER.
- RER captures proteins from cytosol during synthesis
Soluble proteins are fully transported across the ER membrane.
Charcateristsics of transport into ER?
It is cotransational
N terminal signal sequence made first. The signal sequence is cleaved off by signal peptidase.
What are the three ways proteins can move?
-Gated transport, transmembrane transport and vesicular transport.
How does Gated transport work?
Nuclear import: have nuclear pore complexes known as nucleoporins
Nuclear pore receptors: are soluble cystolic proteins and recognise localisation sequences.
Nuclear export: its the other way, they have nuclear export sequences and nuclear export receptors.
How does transmembrane transport work?
It transports stuff across membranes. Cytosol to mitochondria or ER.
How does transport to the ER work?
It is transmembrane transport. It captures proteins from the cytosol during synthesis.
Soluble proteins:
these are transported straight across the membrane as signal peptidase cleaves the signal sequence off so the mature protein goes straight into the cytosol.
Transport into the ER- Cotransational:
1) N terminal signal sequence is formed
2) Signal sequence is recognised by Signal Recognition Protein (SRP)
3) SRP and SRP receptor join on membrane.
4)Polyribosome is attracted to the ER
5) The complex moves to the translocator
6) The SRP-SRP protein complex if release and the growing chain is transferred through the membrane.
How does vesicular transport work?
It relies of budding and fusion
Proteins that are on the lumenal side remain proteins in the lumen even after transport.
ER to Golgi and cell membrane, to and from
What is the secratory pathway?
Goes from ER to Golgi.
It travels through the golgi from secretory vesicle by either constitutive secretion (unregulated membrane fusion) or regulated secretion (regulated membrane fusion by signal transduction)
The exocytosis happens
What happens to protein in the ER?
Glycosylation: where sugars are added to the protein
14 sugars are added en bloc or sugars are transferred from lipid (dolichol) to aspergine residue.
N linked glycosylation (where aspergine = N)
O-linked glycosylation?
Happens in Golgi
Poorly understood
sugars bind to -OH groups of amino acid side chains (ser/thr)
less frequent than N-linked
