M&R 1.2 Membrane Proteins Flashcards
Describe the 2 types of evidence for proteins being present in membranes
- Functional (facilitated diffusion, ion gradients, specificity of cell responses)
- Biochemical (gel electrophoresis, freeze fracture)
How does freeze fracture work?
Lipid bilayer is frozen in ice, then fractured with a knife
The ice crystal will break at the weakest point (between the lamellae of the bilayer)
Proteins will be attached to one lamella and on the other one there will be a corresponding hole where the protein used to be
Describe the 3 ways that PROTEINS can move within bilayers
- Conformational change (e.g. to move things from inside to outside)
- Rotational (movement around its own axis)
- Lateral (e.g. to find a partner - dimerisation of receptors)
What way can proteins NEVER move in bilayers, and why?
Flip-flop
Because it’s so energetically unfavourable - the large hydrophilic moieties would have to pass through the hydrophobic middle of the bilayer in order to switch sides (would require loads of energy!)
What are 3 restrictions on protein mobility in membranes?
- Lipid- mediated effects (in general, proteins tend to be found in lower cholesterol, more fluid regions)
- Membrane protein associations
- with each other (e.g. at synapses)
- with proteins from other cells
- Association with extra-membranous proteins (tethering - e.g. to the BM or to the cytoskeleton)
How are peripheral membrane proteins associated with the bilayer?
They are associated with the membrane, but not within it
Bound to surface by electrostatic & H-bond interactions
How can peripheral proteins be removed from a bilayer?
They can be ‘washed off’ by changes in pH or charge
How are integral proteins associated with the bilayer?
They are transmembranous - they interact extensively with the hydrophobic domain of the bilayer
Can integral proteins be removed by changing pH or charge? If not, how can they be removed?
No
They can be removed by detergents and organic solvents (compete for non-polar interactions and essentially dissolve the bilayer to remove the embedded proteins)
What is the fluid-mosaic model of membrane structure? (Singer-Nicholson model)
Membrane proteins float in a ‘sea’ of lipid
Fluid - because unattached components of the membrane can move around freely
Mosaic - because made up of a patchwork of molecules
Transmembrane spanning domains are often what secondary structure?
Alpha helix
Transmembrane domains are usually ________ amino acids long
18-22 AAs long
Name some AAs that may be found in a transmembrane domain
AAs with hydrophobic R-chains: cysteine, leucine, alanine, phenylalanine
If you know the amino acid sequence of a protein but you don’t know where its transmembrane domains are, what would you use to find this out?
A hydropathy plot
- this maps to find hydrophobic regions of ~18-22AA’s that are likely to be TM domains
Aside from having single/multiple TM domains, describe some other different ways in which proteins can interact with bilayers
- Post-translational lipid modifications (proteins have bits of lipid/FA attached onto their extrinsic area, which lock it into the membrane)
- Dolichol phosphate-linked peptides (protein attached to dolichol phosphate (a carbohydrate) which is in turn attached to a lipid interacting with the membrane)
- Peripheral protein associations (peripheral proteins attached via association with integral transmembrane proteins)
