Lipid Proteins and Cell membrane Flashcards
What are some membrane protein roles?
Structural
Receptors
Transport proteins
Cellto cell communication
What are the types of protein interactions with membranes?
- Integral (or transmembrane):
They span the entire membrane and are permanently attached - Peripheral: do not directly interact with the hydrophobic core of the bilayer; temporarily attached by non-covalent interactions and they associate with one surface of the membrane
- Lipid-linked – protein is bound by a lipid molecule that increases hydrophobicity, allows for control of the protein
Describe integral membrane proteins generally?
They span the entire membrane - permenantly attached (can only be separated by detergents)
Account for 20% of those encoded for by the human genome
Asymmetrically orientated amphiphiles
Usually extend through the bilayer: one end contacts cell interior, the other touches the exterior.
Exposed ends of the protein are hydrophilic
3 types: a-helix, helical bundle and b-barrel
What are the types of integral membrane proteins? functions?
A-helix: recognition and receptors
Helical bundle: Enzymes, transporters and receptors
B-barrel: Transporters
How are the amino acid arranged within the integral membrane proteins?
Alpha helical: Amino acids within the hydrophobic bilayer are non-polar and can cross one or multiple times Beta barrel (porin-fold): amino acids facing the bilayer are hydrophobic; those facing inside can be hydrophilic
Describe alpha helices within the transmembrane proteins?
25 Å in length (around 20 residues) to completely span the membrane
Hydrophobic effect (where the solvent is the lipid bilayer)
Hydrophobic side chains contact the lipid tails - sheliding the polar backbone groups
Hydrophilic loops are exposed outside the membrane
How is membrane topology determined?
Deduced from the amino acid sequence
Hydropathy profile - Assigns a value to each amino acid; positive values for hydrophobic negative values for hydrophilic amino acids.
Identifies long segments of sufficient hydrophobicity to be membrane associated
Calculated for each 20 amino acid stretch
Describe beta-barrels within transmembrane proteins?
Present in outer membrane of bacteria, mitochondria, chloroplasts
8-22 antiparallel b-strands
A b-sheet that forms a closed barrel-like structure
Often in porins (channel forming proteins)
Describe porins that often contain beta-barrels?
General porins have no substrate specificities; selective porins are smaller than general porins, and have specificities for chemical species.
Specificities are determined by the threshold sizes of the porins, and the amino acid residues lining them
How can we purify/separate integral membrane proteins?
Membrane proteins are difficult to purify due to their tight association with phospholipids
Detergents are amphipathic molecules
Hydrophobic part of detergent is attracted to the hydrophobic hydrocarbons
Hydrophilic part of detergent attracted to water
The detergents can be ionic - SDS or non-ionic - Triton-X-100
Ionic will bind to the exposed hydrophobic regions the membrane proteins and the core of the bilayer
Describe peripheral proteins?
Temporarily attached by non-covalent interactions (reversible attachment)
They associate with the surface of the membrane Either attach to integral membrane proteins or form interactions withlipid polar head groups
Purification is mild leaving the membrane in tact - and the liberated protein behaves as a water soluble protein
e.g. phospholipases, spectrin and cytochrome c
Decribe lipid-linked proteins?
Covalently attached lipids that anchor the protein to the membrane
The lipid group can mediate protein-protein interactions and modify the structure/activity of the protein
One protein may have more than one covalently linked lipid group
3 types: prenylated, fatty acylated and GPI anchored
Describe prenylated, lipid-linked proteins?
Covalently attached lipids built from isoprene units
Most common isoprene units = C15 farnesyl or C20 geranylgeranyl residues
Common prenylation sites are in the C-terminal tetrapeptide C-X-X-Y
If Y: Ala, Met, Ser = farnesylated
If Y: Leu = geranylgeranylated
Mainly anchored to intracellular membranes and to the cytoplasmic face of the plasma membrane
Describe fatty acylated, lipid-linked proteins?
Either myristic acid or palmitic acid are linked to membrane proteins
Myristoylation - C14 saturated fatty acid appended via amide linkageto the α-amino group of an N-terminal Gly residue
Stable and irreversible
Used in membrane targetting and signal transduction
Palmitoylation - C16 saturated fatty acid appended via thioester linkage to a specific Cys residue
Occurs on the cytoplasmic face of the membrane
Used in transmembrane signalling
Describe GPI anchored, lipid-linked proteins?
Glycosylphosphatidylinositol-linked proteins
Occur in all eukaryotes and some parasitic protozoa
Only located on the exterior surface of the plasma membrane
What is the cell membrane?
A boundary
Primarily composed of proteins and lipids
Lipids can make up to 20-80 percent of the membrane
Lipids = building blocks
Proteins = carry out most membrane processes
What are the primary functions of membranes?
Keep toxic substances out of the cell
Separate vital metabolic processes conducted within organelles
Contain receptors and channels that allow specific molecules, that mediate cellular and extracellular activities to pass between organelles and between the cell and the outside environment
What is the structure of the cell membrane?
Fluid-mosaic model
Integral proteins ‘float’ in a 2D lipid ‘sea’ in a random or mosaic distribution
Not rigid - bends/flexes in 3D but maintains integrity
Many non-covalent interactions between proteins and lipids hold membranes together
Lipid proteins can diffuse laterally past each other in the lipid matrix
How can the rate of diffusion of proteins within the membrane be determined?
FRAP - Fluoresence recovery after photobleaching
A fluorophore is attached to a membrane component and a laser destroys/bleaches the fluorophore
The rate at which the bleached area recovers its fluorescence indicates the rate unbleached/bleached molecules laterally diffuse in/out of the area
What is the membrane skeleton?
The membrane that covers the cell
It helps define the cell shape
e.g. erythrocyte (RBC) membrane skeleton
Describe the erythrocyte membrane skeleton?
Spectrin protein forms 75% of the erythrocyte membrane skeleton
Two subunits: alpha 280 kDa and beta 246 kDa
Two heterodimers associate in a head-to-head manner (⍺β)2 tetramer
Spectrins form 100 nm-long filaments
Spectrins are cross-linked with cytoskeletal proteins in the erythrocyte membrane
Spectrin also associates with ankyrin
What are lipid rafts?
Membrane subdomains (lateral organisation of lipids and proteins)
Example: Epithilial cells
Apical domain - faces the interior lumen
Basolateral domain - covers the remainder of the cell
The domains don’t mix and have different lipid/protein compositions
What do lipid rafts tend to consist of?
Glycosphingolipids and cholesterol filling the gaps between its tails
Forms an ordered crystalline arrangment
GPI-linked proteins preferentially associate with the rafts
Therefore lipid rafts are involved in intercellular signalling
How are cell membranes synthesised?
They are assembled in the ER as they would dissolve in the cytoplasm
SER - lipids are formed in and inserted into the organelles own membrane
Here flippasescatalyse the rapid translocation of phospholipids across the ER membrane, resulting in even growth of both halves of the bilayer
RER - involved in protein synthesis and folding
This is the major branch point for the traffic of proteins
