Transport

Question 1

What is GPCR?

Answer 1

It is the largest class of human cell receptors. Smell, taste, vision, hormone detectors. It has 7 transmembrane domains.

Question 2

What is a G protein

Answer 2

Peripheral membrane protein

Question 3

What is the structure of a G protein

Answer 3

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.

Question 4

What is the G protein activation cycle?

Answer 4

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.

Question 5

What does PKA do?

Answer 5

It is a kinase so it can modulate transcription (gene expression)
Phosphorylates many targets.

Question 6

What is the structure of PKA and what does it require?

Answer 6

It has 2 regulator subunits and 2 catalytic subunits. Regulator subunit has cAMP receptors. When bound, the regulator and catayltic subunits separate.

Question 7

What does Gq do?

Answer 7

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.

Question 8

What does IP3 do?

Answer 8

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.

Question 9

What are the cellular effects of calcium?

Answer 9

Activates calmodulin which binds to calcium ion.

Question 10

What does RTK stand for?

Answer 10

Receptor tyrosine kinase.

Question 11

How does RTK binding work?

Answer 11

When a ligand binds to the kinase, they dimerise and phosphorylate each other.

Question 12

What is the MAPK pathway?

Answer 12

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.

Question 13

How receptor signalling is good?

Answer 13

1. endosomes: low pH so ligand dissociates.
2. lysosome: entire receptor ligand complex is broken down in lysosome.
3. receptor inactivation: phosphatases.
4. Inactivation of signalling protein
5. production of inhibitory proteins and transcription regulators.

Question 14

How are proteins moved from one place to another?

Answer 14

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.

Question 15

What is transmembrane transport?

Answer 15

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.

Question 16

Charcateristsics of transport into ER?

Answer 16

It is cotransational
N terminal signal sequence made first. The signal sequence is cleaved off by signal peptidase.

Question 17

What are the three ways proteins can move?

Answer 17

-Gated transport, transmembrane transport and vesicular transport.

Question 18

How does Gated transport work?

Answer 18

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.

Question 19

How does transmembrane transport work?

Answer 19

It transports stuff across membranes. Cytosol to mitochondria or ER.

Question 20

How does transport to the ER work?

Answer 20

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.

Question 21

How does vesicular transport work?

Answer 21

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

Question 22

What is the secratory pathway?

Answer 22

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

Question 23

What happens to protein in the ER?

Answer 23

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)

Question 24

O-linked glycosylation?

Answer 24

Happens in Golgi
Poorly understood
sugars bind to -OH groups of amino acid side chains (ser/thr)
less frequent than N-linked

Question 25

What do chaperone proteins in the ER do?

Answer 25

They bind onto an unfolded protein and ensure quality of protein is maintained.

Question 26

What do protein coats do?

Answer 26

They help capture molecules for onward transport and shape membrane

Question 27

Give examples of protein coats?

Answer 27

Clathrin+ adaptin1= golgi—> lysosome
Clathrin+ adaptin2= plasma membrane—> endosomes
COPI
COPII
Both COPS are to the ER from Golgi or other way.

Question 28

How do Rab GTPases and SNARE’S work?

Answer 28

1. Vesicle has cargo loaded, Rab GTP is attached. V-snare protein attaches.
2. Vesicle is attracted to Rab-tethering protein/RAB effector which pulls the vesicle closer
3. T-SNARE on membrane winds together with the V-SNARE protein and pulls the vesicle close to allow it to fuse to the membrane. The contents are exocytosed into vesicle.

Question 29

What does vesicle fusion depend on?

Answer 29

SNARE proteins

Question 30

Early vs late endosomes?

Answer 30

Early Endosomes (EE) sort cargo for onwards destination
Late Endosomes (LE) / multivesicular bodies (MVB) prepare cargo for degradation/secretion
Lysosomes degrade cargo

Question 31

What are the properties/uses of Intermediate filaments?

Answer 31

Uses: for cells that have to undergo pressure (not movement). Great strength, enables cells to withstand mechanical stress that occurs when stretched. Strengthens nuclear membrane, producing nuclear lamina. Forms desosomes between cells (bits that hold the cells together).
Structure: Alpha monomeric strands twist with other strands to produce a DIMER and binding is non-covalent. Structure is rope-like. Diameter is 10nm. Plectin (accessory protein) required to reinforce filaments.

Question 32

What is the structure and function of the microtubule?

Answer 32

Structure: Microtubules are polarised throughout the cell. They are long. hollow and stiff. They are made of alpha and beta tubulin by non covalent bonds and 25nm diameter. They rupture when stretched. They are polar (alpha is minus end, beta is plus end)
Function: Provides motorway system for vesicles and other stuff. Allows mitotic division. Has dynamic instability- microtubule strands grow out of centromeres in different directions, so if the direction is not required, the microtubule quickly falls apart and a new strand in a different direction is made.

Question 33

What three main filaments is the cytoskeleton made of? How are they distributed across the cell?

Answer 33

Intermediate filaments- from nucleus to rest of the cell
Microtubules-spider out from nucleus to the rest of the cell
Actin filaments- around the cell and microvilli.