1.2 - Membrane Proteins, Membrane Asymmetry And The Cytoskeleton Flashcards
What is the biochemical evidence for membrane proteins?
Membrane fractionation and gel electrophoresis
Freeze fracture - fracture between gaps of bilayer seen under a microscope
What is the functional evidence for proteins in the membrane?
Facilitated diffusion
Ion gradients
Specificity of cell response
What are peripheral proteins?
Bound to surface of membrane
Through electrostatic and hydrogen bond interactions
Removed by changes in pH or ionic strength
What are integral proteins?
Integrated extensively with hydrophobic domains of the lipid bilayer
Cannot be removed by manipulation of pH or ionic strength
Removed by agents that compete for non-polar interactions, e.g. Detergents
What are transmembrane domains?
Parts of a protein that span the whole membrane
The R-groups of AA residues are largely hydrophobic
Often have an alpha-helical structure
Some proteins have multiple transmembrane domains
How are proteins associated with bilayers?
DOLICHOL phosphate -linked polypeptides
Peripheral protein associations
Post-translational lipid modifications
What restricts the mobility of proteins?
Aggregates - if the proteins are too big
Tethering - with basement membrane or cytoskeleton
Interaction with other cells - tethering between cells
What is the significance of the Cytoskeleton in an erythrocyte?
Maintains the biconcave disc shape of erythrocytes
Describe the membrane skeleton
Made up of spectrin and actin molecules
Attached to membrane by Ankyrin and Glycophorin adapter proteins binding to band 3 and band 4 respectively
What is Hereditary Spherocytosis?
Depletion of spectrin
Round erythrocytes
Less resistant to lysis so cleared by spleen
What is Hereditary Elliptocytosis?
Defect in spectrin molecules
Unable to form hetertetramers
Fragile elliptoid cells
How are membrane protiens inserted into the membrane?
Free ribosome initiates synthesis from free mRNA
Hydrophobic N-terminus signal sequence produces
Signal recognised and attaches to SRP
Protein synthesis stops
SRP bound to ribosome binds to SRP receptor on ER surface
SRP dissociates
Protein synthesis continues and the protein enters the ER via a pore in the membrane
Signal cleaved by signal peptidase
Once the ‘stop transfer’ signal enters the ER membrane it remains there
The rest of the protein is translate outside of the ER is the cytoplasm so the protein spans the membrane
What is the significance of membrane asymmetry?
The orientation is important for function e.g. insulin receptor must have site facing extracellular space
How is correct orientation of membrane proteins achieved?
Usually there is a signal peptidase cleaver site that cleaves the signal as it enter the ER so the N-terminus enters the ER lumen and the C-terminus remaisn outside the ER.
Without this signal peptidase the C-terminus would enter the ER lumen.
