Membrane proteins Flashcards
Membrane proteins have 3 structures
alpha-helix
beta-barrel
amphipathic helix
What can membrane proteins be attached to ?
Other proteins or lipid anchored
Hydropathy plots
Indicate the hydrophobicity of protein segments - used to predict trans-membrane domains.
What do trans-membrane proteins act as?
Transporters
Receptors
Anchors
Enzymes
Two ways that proteins can interact with membranes
single pass
multi pass
4 examples of peripheral membrane proteins
Lipid associated
Lipid modified anchor protein
GPI anchored
Proteins attached to integral proteins
Why is the alpha helix a common structure for trans-membrane proteins
H bonds are stable in the backbone due to a lack of water
Hydropathy
A calculated measure of the energy needed to pass a segment of polypeptide into water from a solvent.
Multi pass transmembrane proteins
Some proteins have multiple transmembrane alpha helices
Gly is common in alpha helices of multipass transmembrane domains and can allow for flexibility and tight packing.
Describe Beta-Barrel
Some proteins have beta-barrel structures which consist of antiparallel beta strands. Hydrogen bonds occur between barrels. Pore is in the middle with hydrophilic side chains in the middle and hydrophobic side chains facing outwards.
Studying trans-membrane domains of SER & RER
Obtain cell membrane compartment, homogenise then place in test tube with gradient of increasing sucrose solution.
Smooth microsomes have a low density so stop at a low sucrose solution.
Rough microsomes have a high density so stop sedimenting at a high sucrose solution.
What is the process of characterising trans-membrane proteins?
Solubilisation = detergents Isolation = chromatography Characterisation = gel electrophoresis Functional/structural = incorporation into artificial membranes
How can you experimentally confirm trans-membrane proteins?
Fractionate cell to confirm that the protein is in a membrane fraction.
Wash with salt e.g. sodium chloride
Physically break the membrane open by grinding up after freezing- soluble content released from the cell
Solubilise protein with detergent
What are detergents?
Amphipathic molecules that incorporate into a membrane to disrupt the structure.
What shape do detergents form?
Micelles in bilayers
How do detergents exist at low and high concentrations?
Low = monomers and insert into a membrane High = form micelles and disrupt the membrane
What are three examples of cytosolic lipid modifications?
Myristoyl anchor - amide link between terminal amino group and myristic acid
Palmitoyl anchor - thioester linkage between cysteine and palmitic group
Farnesyl anchor - thiether link between cysteine and prenyl group
Ras protein
Signalling protein that is a small GTPase. It is switched on by incoming signals and switches on other proteins involved in cell growth, differentiation and survival.
