Lecture 12: Membrane Targeting Protein Flashcards
What are type 1 proteins ?
Have a single transmembrane span. The N-terminal is in the extracellular domain and the c-terminal is in the intracellular domain. Have a cleavable n-terminal leader sequence
What are type 2 proteins ?
Lack a leader sequence. C terminal is now extracellular and the n-terminal is cytoplasmic. Have a single transmembrane span
What is the type 3 protein ?
Single transmembrane span. The n -terminal is extracellular and the c-terminal is on the cytoplasmic side. No n-terminal cleavable sequence
What is a type 4 protein ?
Multi span, can have terminals in both cytoplasms on both extracellular
What are type 5 and 6 protiens ?
Have in addition to or covalent modification with lipid. Type 6 is anchored to membrane by lipid
What are the other types of protein ?
- Beta barrel
- Proteins that only associate with membrane
- Proteins that are soluble
What do spanning regions look like ?
Consist of 20-25 hydrophobic amino acid residues that form a right hand alpha helix that crosses the bilayer
Where are all nuclear encoded proteins synthesised ?
In cytoplasm
Where are membrane proteins directed for translocation ?
Endoplasmic reticulum
Every protein has its own signal, what is the only common feature ?
A central segment of 6-20 hydrophobic residues
Where does translation of mRNA begin ?
Ribosome
How does a signal sequence direct a protein to ER for translocation through membrane ?
- Translation
- Signal sequence emerges and is recognised by SRP
- SRP biinds to signal sequence enabling interaction with ER membrane
What does docking require ?
GTP binding
What do interactions betwen the signal sequence and sec61alpha lead to ?
Stabilising interactions of ribosome and translocon and leads to insertion of the signal sequence with the translocon
What are transmembrane domains recognised by ?
Translocon because of their hydrophobic nature
How do transmembrane domains exit the translocon ?
By moving laterally through a lipid protein interface and become integrated into the bilaer
What is a frequent feature of stop transfer ?
Presence of basic residues immediately proxiaml to the cytoplasmic side of the sequence
What are signal sequences used for ?
Targeting to and translocation across the membranes of other organelles
lipid membrane bilayers are required by cells to prevent what ?
- Loss of molecules by simple diffusion that a cell wishes to retain
- Entry of molecules by simple diffusion that a cell wishes to exclude
Why are membrane transport systems required by cells ?
Hydrophobic barrier properties of lipid bilayers prevent the free diffusion of all charged and hydrophilic molecuels that a cell wishes to either to take up or to discard
What are the characteristics of carrier mediated transport ?
- Saturable
- Specific
- Competition
- Temperature
- Mode
What is valinomycin ?
An ionophore produced by bacteria that specifically allows movement of potassium
What are the types of active transport ?
- Coupling via ion gradients
- ATP coupled
- Group transfer
