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Membrane proteins
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In depth cell signalling and physiology notes, created using synchronous and asynchronous material.
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Membrane proteins.Functions of membrane proteins
o Transport of nutrients
Glucose transporter
• Passage of water
o Aquaporins
• Selective transport of molecules
o Primary/Secondary Transporters
• Maintenance of proper ionic composition inside the cell
o Aquaporins, Primary/Secondary transporters
• Reception of signals from the extracellular environment
o GPCRs (yay), LGIC, VGIC
• Expression of cell identity (self vs non-self)
o MHC complexes
• Physical and functional connection with other cells or extracellular matrix (in
multicellular organisms)
o Tetraspanins
• Membrane proteins control what gets in and what gets out of a cell.
• This can range from nutrients and water and small molecules.
• To receptors which transduce signals from the exterior surface of the cell into the
interior
• Proteins such as the MHC complex can let the immune system know what is self vs
non-self.
• As well as forming physical connections between two adjacent cells in the form of
tetraspanins.
• Membrane proteins are there to control information.
• Also control the nutrients that come into and out of the cell and form structures with
adjacent cells so that you can form nicely tightly packed tissues.
Various ways proteins associate with the lipid bilayer.
• How membrane proteins are attached or embedded within the bilayer.
• We have intrical membrane proteins, peripheral membrane proteins and also
specialist ones.
Integral membrane proteins.
• Traverse both leaflets
• Single transmembrane (TM) helix
• Multiple TM helices
• Multiple βsheets folded into a TM β-barrel
• Integral membrane proteins:
• We have singular alpha helixes
• We have multiple helixes, which form in a bundle. (this is the case for lots of
membrane proteins, such as iron channels, GPCRs, to voltage gated iron channels)
• Beta barrels- instead of alpha helixes this is collections of beta sheets, which
condense into these beta barrel structures.
, • These are more seen in bacteria, and it allows the wholesale influx or efflux of small
molecules and nutrients into and out of the cell.
Peripheral membrane proteins.
• Protein interacts with a single leaflet; does not traverse bilayer
• are permanently attached to lipid membrane from only one side and do not span
across the whole bilayer
• Noncovalent interactions;
• ionic charges,
• e.g. amphipathic helix
• In peripheral membrane proteins:
• Either part of the membrane is associated with the membrane.
• Or part of it inserts slightly into the membrane.
• But the big difference between peripheral membrane proteins and integral
membrane proteins is the amount of protein that transverses.
• In integral membrane proteins part of it transverses the lipid bilayer.
• Whereas in peripheral membrane proteins we do not see them transverse the whole
of the bilayer, they may only insert themselves into the inner leaflet of the bilayer.
• They do this through a variety of mechanisms, normally it is through ionic
interactions between proteins.
• At the top we have a helix which may have a whole series of charges on it, those
charges then interact with those positive/negative charges in the head groups.
• Lipid head groups play a large part of this, there are different charges and chemistry
going on within the headgroup.
• You can have some specificity of interaction not only from the protein but depending
on which lipids are actually expressed within the lipid bilayer.
Lipid anchored membrane proteins.
• A fatty acid chain or a prenyl group in cytosolic monolayer e.g. palmitoyl, farnesyl
fatty acids
• Also found on membrane proteins with multiple TM helices
• An oligosaccharide linker to phosphatidylinositol in outer leaflet
• GPI anchor (Glycosylphosphatidylinositol)
• We have proteins with fatty acid attachments.
• This is the outside of the cell, this is the extracellular surface
• Membrane has a covalently binded fatty acid tail,which then anchors it into the
membrane.
• Outside of the cell we have a similar mechanism, we have a fatty acid
(phosphatidylinositol), which is linked to a series of sugars to our protein of choice.
• So we have covalent linkage between the protein and the lipid.
• lipid is able to insert itself into the bilayer.
• On the exterior surface of the cell, we have a lipid and then that is then linked
through a series of sugars to our particular membrane protein of choice.
Why lipid anchors?
• N-terminal FA anchors
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