2. Membrane Proteins, Membrane Asymmetry and the Cytoskeleton Flashcards
How does freeze fracture provide supporting evidence of proteins in the bilayer?
Ice crystals break around the weakest points of the structure - between two lamellae. The protein either gets pulled away with the top lamellae or left in the bottom one - leaving a hole in the top one.
What motions of proteins are permitted in the bilayer?
Conformational changes, rotational and lateral. (No flip flop movement due to energy unfavourability).
How is protein movement in the membrane restricted?
Aggregates - clumps of protein mean the proteins are fixed and can’t move.
Tethering.
Interactions with other cells.
What are the two types of membrane proteins by positioning?
Peripheral - bound to the surface by electrostatic and hydrogen bond interactions, removed by changes in pH or ionic strength.
Integral - embedded in the bilayer and interact extensively with hydrophobic domains of pH and ionic strength, removed by agents that compete for non-polar interactions.
How many amino acids are needed to span the width of the bilayer in transmembrane polypeptides?
18-22.
What do hydropathy plots show?
How many transmembranous domains there are. This equates to the number of hydrophobic peaks on the plot.
What is membrane protein topology?
The orientation of C and N-terminals.
What are the different ways in which proteins can be associated with the bilayer?
Single or multiple transmembrane domains, dolichol phosphate-linked polypeptide, peripheral protein associations and postranslational lipid modifications.
What is the erythrocyte cytoskeleton?
A network of spectrin and actin molecules. Spectrin is a long, floppy rod- like molecule. a and B subunits wind together to form an antiparallel heterodimer and two heterodimers then form a head-to-head association to form a heterotetramer of a2B2 . These rods are crosslinked into networks by short actin protofilaments (~14 actin monomers), and band 4.1 and adducin molecules which form interactions towards the ends of the spectrin rods. The spectrin-actin network is attached to the membrane through adapter proteins. Ankyrin (band 4.9) and band 4.1 link spectrin and band 3 protein and glycophorin A, respectively. Attachment of integral membrane proteins to the cytoskeleton restricts the lateral mobility of the membrane protein.
Describe hereditary spherocytosis.
Spectrin is depleted by 40-50%. Erthrocytes round up instead of being biconcave so are less resistant to lysis, and are cleaned by the spleen. This leads to less red blood cells - anaemia.
How is hereditary spherocytosis treated?
With blood transfusions to replace the red blood cells.
What is hereditary elliptocytosis?
defect in spectrum molecules so they’re unable to form heterotetramers. They form fragile elliptoid cells that have an inability to carry sufficient oxygen.
What is the normal membrane protein orientation?
N terminal on ER lumen side, C on cytoplasmic side.
What happen to membrane protein orientation in the absence of a signal peptidase cleavage site?
The C-terminal goes to the ER lumen and N-terminal to the cytoplasm.
What evidence is there for proteins in membranes?
Functional evidence - facilitated diffusion, ion gradients and specificity of cell responses are all controlled by proteins so proteins must be there.
Biochemical evidence - membrane fractionation and gel electrophoresis and freeze fracture.
