The secretory pathway Flashcards
How is the lipid symmetry achieved/maintained - ER membrane
Start with a lipid bilayer
Phospholipid synthesis adds to cytosolic half of the bilayer
Scramblase (equilibrates lipids) catalyses flipping of phospholipid molecules so that there is symmetric growth of both halves of the bilayers
How is the lipid asymmetry achieved/maintained - plasma membrane
start with asymmetric lipid bilayer of plasma membrane
then delivery of new membrane by exocytosis causes a disruption of asymmetry
Flippase catalyses flipping of specific phospholipids to cytoplasmic monolayer and restores asymmetry
Energy dependent “flippase” ensures membrane asymmetry maintained
Asymmetry of phospholipids of RBD membrane
Negatively charged phospholipids (phosphatidylserine) mainly in the cytosolic leaflet
Transfer of phosphatidylserine from the extra cellular leaflet of the plasma membrane to the cytosolic leaflet
Flippase is generally more favoured/dominant
Aminophospholipid translocase transfers phosphatidylserine from outer leaflet of plasma membrane to inner leaflet
The scramblase abolishes asymmetry
Net equilibrium favours translocase under normal conditions
Scotts disease: deficiency in scramblase, leads to increased bleeding
ER and golgi apparatus
ER - structure and basic functions
Vesicle transport
Golgi - structure and basic functions
Composition membrane bound organelles different between different cell types
Proteins begin being made in cell by expanding ER
Plant cell walls define variety structures and functions
Where is ER and golgi in cell
Er is dynamic network that is continuously breaking and reforming, can expand for large distance throughout cell
Golgi generally close to nucleus
Er
Is connected to nuclear envelope
Forms hollow tubes and flattened sacs, chambers are cisternae
Two types of ER and consist of different domains
- Rough (RER) outer membrane covered in ribosomes, associated with polyribosome
- Smooth (SER) no ribosomes
Functions of ER
Quality control
Synthesis
Storage
Detoxification
Quality control
Newly made membrane and secreted proteins need to be translocated into ER
Ribosome synthesise protein and opening/closing pores allow protein into lumen of RER
Chaperone proteins help newly synthesised linear sequences of amino acids fold correctly into tertiary and quaternary structures
BiP very present chaperone protein in lumen ER
FOlded correctly then protein can exit Er, if folded incorrectly can’t exit through pore and get removed from ER and broken down in cytoplasm
SER
No ribosomes associated with it
Responsible for:
- Phospholipid and cholesterol synthesis
- Steroid hormone production
- Synthesis and storage of glycerides
- Synthesis and storage of glycogen
- Important role as calcium store
Calcium signalling in Acinar cells
Zymogen granules contain enzymes important for digestion
Stimulation -> Ca2+ release -> vesicle fusion -> enzyme release
Pancreatitis
Can result from Ca2+ overload
Alcohol or biliary disease:
- Aberrant activation of intracellular trypsin
- Vacuolation and necrosis
Failure to maintain calcium homeostasis may be due to part to failure of Ca2+ pumps at PM
Principles of vesicular transport - transport from ER to golgi
Transport between ER (donor compartment) and golgi (target compartment) in form vesicle and tubules
Vesicles bud off ER and received by Golgi
Anything in lumen or membrane of vesicle transported to new compartment, nothing lost during transport
Fusion maintains asymmetry of membrane
Transport vesicles have high SA:V, this increases specificity of cell
Cargo incorporated into coated vesicles
Need to look at diagram
Forward - anterograde direction
Backwards - retrograde direction
Vesicles are “coated”
Area of membrane forming vesicle coated with cage specialised proteins
Coat aids formation vesicle, but has to be discarded before vesicle can fuse with target compartment
Three types vesicle coat:
- Clathrin (identified in 1980’s)
- COPI - important for moving forward and backwards through golgi
- COPII - taking new proteins from ER to golgi
Underlying way that coats formed is same they’re just different proteins
How do vesicle reach correct target - role of SNAREs
SNARE - soluble N-ethylmaleimide-sensitive factor adapt to protein receptor
SNAREs are integral membrane proteins
Ensure that right vesicle fuses with right organelle
Two types:
- v-SNAREs: vesicle SNAREs - found in vesicle membrane
- t-SNAREs: target SNAREs - found in membrane of target membrane
In nerve terminals, SNARE complex involved in docking synaptic vesicles is helical bundle consisting of 3 components; v-SNARE synaptobrevin, t-SNARE syntaxin and t-SNARE Snape 25
Golgi apparatus
Composed of flattened discs - also called cisternae
Typical Golgi apparatus normally consist of 5-6 cisternae
Tends to lie near nucleus
Cisternae communicate with ER and cell membrane by use vesicles and tubules
Has 3 primary functions:
- Modification and packaging of secreted proteins
- Renewal and modification of plasma membrane
- Delivery of material to other organelles, especially the endocytic pathway
IS highly structures organelle
Vesicles released from ER bind to cis face and then processed and leave from trans face
Many modification processes take place in Golgi
Each modification process takes place in specific region (Look at diagram)
The trans-Golgi network is major sorting station for newly-made proteins
Plasma membrane and secreted proteins are delivered to the cell surface by the constitutive secretory pathway in non-polarised cells
Movement of newly synthesised protein through the secretory pathway
The trans-Golgi network is a major sorting station for newly-made proteins
