Transport across biological membranes Flashcards
Describe the synthesis of phosphatidylcholine
- Fatty acids synthesised in cytosol are transported to ER by FABP (fatty acid binding protein) (highly hydrophobic so need to be transported in an aqueous environment)
- FAs embed in the membrane and in a succession of steps glycerol, phosphate and choline added (this happens in the outer cytosolic)
Where does phospholipid synthesis occur?
in the outer cytosolic leaflet of the ER membrane
How are phospholipids distributed equally between the outer and inner membranes of the ER membrane?
The ER enzyme scramblase catalyses a flip-flop movement (e.g trans-bilayer movement)
What happens to newly synthesised membranes?
Transported to the plasma membrane and to other organelles
What does the inner luminal leaflet of the ER become on the plasma membrane?
the extracellular leaflet of the plasma membrane
How does the inner luminal leaflet of the ER become the extracellular leaflet of the plasma membrane?
A vesicle of membrane buds off the ER.
The vesicle fuses with the membrane in a way that the inner luminal leaflet is now on the outside
Explain how the asymmetric distribution glycolipids occur in the plasma membrane?
- Enzymes that add sugar are only found in the inner luminal leaflet of the ER
- So when the vesicles fuse with the plasma membrane it ends up on the outside of the cell
Where does glycosylation occur?
In the lumen of the ER/golgi
Where are the glycolipids located?
On the extracellular leaflet of the plasma membrane
Based on sphingosine
Where are integral membrane proteins found?
Embedded into the membrane
Where are peripheral membrane proteins located?
On the cytoplasmic side or extracellular side when in association with integral membrane proteins
What are the three types of transmembrane protein?
Single pass transmembrane protein (alpha helix)
Multi-pass transmembrane protein (multiple α-helices)
Multi-pass transmembrane protein (B-sheets rolled up into a B-barrel)
What do soluble proteins for export contain at their N-terminal end?
A 15-20 amino acid hydrophobic signal peptide
How are integral transmembrane proteins inserted into the membrane?
- Signal peptide directs the growing polypeptide chain as it emerges from the ribosome to a translocator in the ER membrane
- The growing polypeptide chain is threaded through the membrane (translocation)
- Signal peptide is cleaved by signal peptidase and newly synthesised protein is released into ER lumen
How does the integration of a single pass transmembrane protein occur?
- Some proteins contain both a signal (start-transfer) peptide and a stop-transfer sequence
- Signal peptide directs the growing polypeptide chain to the translocator and the growing chain is threaded through membrane
- When translocator interacts with stop-transfer sequence translocation ceases and the translocator discharges the protein laterally into the bilayer
- Signal peptide is cleaved
How does the integration of a double pass transmembrane protein occur?
- Double pass membrane spanning proteins have an internal start-transfer peptide and a stop-transfer sequence
- Polypeptide synthesis is initiated in the cytoplasm, signal peptide directs the growing chain to the translocator and chain is threaded through
- When the translocator interacts with the stop-transfer sequence translocation stops and translocator discharges protein into bilayer laterally
- Signal peptide is not cleaved
What is the difference between integration of double membrane pass proteins and single membrane pass proteins?
In double pass the signal or start-transfer sequence is internal and its not exactly at the N terminus of the polypeptode
In double signal peptidase is not involved
Where are sugar residues localised on glycoproteins?
On the extracellular side of the plasma membrane
what type of amino acids are sugar residues added to?
Ones that are exposed to the lumen of the ER
Why are glycoproteins and glycolipids important for? (3 roles)
Cell recognition
Inflammatory response
Protection
What is the glycocalyx?
A carbohydrate rich layer surrounding the cells which is composed of glycolipids and glycoproteins
What is the role of the glycocalyx?
It protects cells against chemical and mechanical damage
What is the nuclear envelope continuous with?
ER membrane
What does the nuclear envelope consist of?
Inner and outer membrane
What is the nuclear envelope penetrated by? And what is the role of the thing penetrating it?
Nuclear pores to allow bidirectional exchange between nucleoplasmic and cytoplasmic compartments
How many nuclear pores are there per nucleus?
3000-4000
Describe the nuclear pore complex
- Largest protein complex in the cell (50MDa)
- Greater than 400 individual protein complexes
- 30 distinct proteins (nucleoporins)
- Both integral and peripheral membrane proteins involved
What does the enzyme flippase do?
Flips the Phosphatidyl Ethanolamine and Phosphotidyl Serine from the extracellular leaflet to the cytosolic leaflet creating an asymmetric bilayer.
Describe the steps in membrane synthesis
- Phospholipid synthesis occurs exclusively in the outer cytosolic leaflet of the ER membrane
- ER enzyme Scramblase catalyses flipi-flop reaction so phospholipids distrubute equally between the membranes.
- Newly synthesised membrane is transported to the plasma membrane and other organells.
- the inner luminal leaflet of the ER becomes the extracellular leaflet of the plasma membrane.
- Plasma membrane enzyme flippase flips PE and PS to the cytosolic leaflet creating the asymmetric bilayer.
