Lecture 9 - Membrane Proteins and Transport Flashcards
Peripheral membrane proteins attach/detach to the surface of a membrane. Usually bind to the phosphate heads. 3 main structural features?
- Polar pocket/groove that recognises a specific ligand.
- Protein protrusion that penetrates the phosphate head region and anchors it to the membrane. Some can be hydrophobic and penetrate into the membrane.
- Cluster of basic residues that bind anionic (neg) phospholipid heads.
Lipid binding domain examples.
PH domain - found on the protein PLC, binds phosphatidylinositol 4,5 biphosphate.
BAR domain - curved, helps to shape the membranes. Binds to phosphate heads.
The type of protein/protein domain that associates with the membrane depends on?
- Type of membrane and the types of phosphoinositide present.
- [Ca2+]
- Shape of the membrane.
Proteins can also be ANCHORED to the membrane. Two types?
Lipid anchors and GPI anchors.
Examples of lipid anchors.
Small trimeric G proteins, GPCR C-terminal domains and SNAREs.
Most lipid anchors are non permenant. Reversible. Name the 3 types of lipid anchors.
Palmitoylation - palmitoyl groups added to the Cys or Ser.
Farnesylation - addition of farnesyl group on C-terminal Cys.
Myristoylation - addition of M-meristoyl-group on N-terminal gly. Permanent.
What does GPI stand for in GPI anchor?
GPI = Glycosyl Phosphatidylinositol, a glycolipid. The glycolipid attaches to the C terminus of the protein to anchor it.
Where are GPI anchors found in comparison to lipid anchors?
GPI anchors = outside of the membrane.
Lipid anchors = cytosolic side.
Are GPI anchors more or less permanent than lipid anchors?
More permanent.
Example of a GPI anchor?
VSG coat of trypanosomes.
GPI anchors tend to move the proteins they are attached to into…
Lipid rafts.
Integral membrane proteins are also called…
Transmembrane proteins.
Basic features of integral membrane proteins.
Helical structure - alpha helix of 20 hydrophobic AAs span the membrane. Outside of the structure there are hydrophilic AA side chains.
There are 4 categories of integral membrane proteins. Name these.
Type I - C terminus on cytosolic side. N terminus on the extracellular side.
Type II - opposite way round.
Type III - short N terminus
Type IV - membrane proteins with more than one TM domain.
Detergent can be used to disrupt the membrane through solubilization. This can be used to study TM proteins. The amount of detergent added determines the effect. Explain this.
- Low amount = detergent inserted into the bilayer.
- Medium amount = detergent doped membrane will break up bilayer.
- High amount = removes all the lipid from the protein, forming lipid-detergent mixed micelles and separate protein detergent complexes.
Some regions resist solublization. Example?
Lipid rafts.
What do lipid rafts consist of?
Sphingomyelin and cholesterol. H bond forms between the OH of the cholesterol to the amino group of sphingomyelin.
The FM model is not suitable for lipid rafts. Why?
Solid.
What do lipid rafts do?
- Signalling - can bring together signalling components, increasing their conc + efficiency.
- Alter conformation of proteins - the TMD will have a slightly different conformation in a raft bc of its rigidity.
- Host/pathogen interactions - provides structured environment for pathogen binding, virus budding.
Give an example of lipid asymmetry.
Phosphatidylethanolamine (PE) found mainly on the inner leaflet. Sphingomyelin mainly found on the outer leaflet.
The cytoplasmic inner leaflet contains…
More (-) charged amino phospholipids and PE.
There are two ways in which lipid asymmetry is developed. Explain these two mechanisms, and any problems that may arise.
- Flip-flop diffusion - does not occur spontaneously. Energetically unfavourable. Phospholipid must pass through hydrophobic region.
- Lateral diffusion - energetically favourable.
Enzymes that catalyse flip-flop diffusion?
- Flipases - outer to INNER.
- Flopases - inner to OUTER.
- Scramblases - both.
How are other solutes transported?
Pumps (primary transporters), carriers and channels.
Pumps
- Active transport, ATP.
- Often operate in one direction.
- Generate large gradients.
- E.g. ABC transporters.
Two types of carriers. What are they? Function?
- Passive transport of a single solute, these are called uniports.
- Coupled transporters - transfer of one solute depends on the transport of a second. Symporters (co-transporters) = same direction. Antiporters = opposite direction.
Channels (ions and water)
- Transport is passive, relies on ion gradient.
- Ion selectivity and open/close kinetics.
- Low affinity - do not bind the solute.
Which of the three transport mechanisms have the greatest turnover rate?
Channels.
Which of the three transport mechanisms have the highest density?
Pump density is the greatest because they have the slowest turnover rate.
ABC transporters are a family of pumps. They are ubiquitous. What does this mean?
They are found in all kingdoms of life.
How do ABC transporters (pumps) function?
The nucleotide binding domains (NBDs) are pulled together by ATP binding. ATP hydrolysis then flips the pore from internal to external access. Alters affinity.
Phosphate release opens NBDs again, cycle repeats.
Water is hydrophilic. How is it quickly transported through the membrane?
Aquaporins = fast. Highly selective = channel diameter is perfect for water transport.
