Membrane Proteins and Membrane Fluidity

Membrane Proteins and Membrane Fluidity

• Membranes are a mosaic of proteins embedded and dispersed in the phospholipid bilayer and are somewhat fluid.
• Glycans are responsible for recognition and signalling.
• Glycans anchor cells in their environment and anchors ensure cellular membrane movement to the cytoskeleton.
• Lipids and proteins can move around within the membrane.
• Lipids diffuse laterally around all the time.
• Proteins not involved in anchoring also diffuse, which is important for the protein to change shape for function.
• Proteins need to transmit signals.
• Transport occurs across by diffusion or using a transporter.
• Vesicles need to bud off and fuse.

Measuring Rate of Lateral Diffusion

• Fluorescence Recovery After Photobleaching (FRAP) is a technique used to measure the rate of lateral diffusion in a membrane.
• The membrane is labelled with fluorophores that shine and emit light.
• Intense light bleaches the fluorophores in a specific area.
• The rate at which fluorescence returns to the bleached area is measured to determine the rate of diffusion.

Fluidity

• Biological membranes have constant movement within the bilayer.
• The rate of membrane lipid movement can be measured.
• Rotation and flexion occur at a high rate.
• Lateral diffusion occurs at ~2 μm per second, about one tenth the length of a mammalian cell.
• Transverse diffusion (flip-flop) occurs rarely, about once every 3 days, often requiring enzymes.
• Proteins are similar, but generally move slower than lipids in the membrane.

Temperature and Membrane Fluidity

• Membrane fluidity is affected by temperature.
• At low temperatures, the membrane is in a more ordered, crystalline state.
• As temperature increases over a short interval, the membrane transitions to a more disordered, fluid state (gel-like).
• Tm is the halfway point in this transition.
• Too much fluidity leads to a disordered membrane and increased permeability.
• Being too solid slows down movement.

Increased Fluidity

• As temperature increases, fluidity is increased because fatty acid tails can flop around a lot more.
• Lipid molecules move faster to become more permeable.

Decreased Fluidity

• As temperature decreases there is a decreased fluidity.
• Lipid molecules move more slowly and become gel-like.
• The membrane becomes less permeable.

Lipids and Membrane Fluidity

• Membranes contain a mixture of lipids.
• Kinks result from cis formation
• Unsaturated lipids cause increased fluidity.
• Short chains allow fewer interactions between lipids.
• Decreased fluidity caused by saturated chains.
• Long chains and low temperature.

Regulation of Composition

• Organisms regulate their lipid composition such as organisms that cannot internally regulate their temperature.
• Short, unsaturated fatty acids predominate at low temperatures.
• Conversely, long, saturated fatty acids predominate at high temperatures.
• Plants have sensors in the plasma membrane to detect change.
• Fluidity increase indicates increased temperature.
• Thus the plant can prepare for possible heat stress.

Cholesterol and Membrane Fluidity

• Cholesterol is found only in animal cells.
• At warm temperatures (e.g., 37°C), cholesterol makes the membrane less fluid by restraining phospholipid movement
• At lower (cool) temperatures, cholesterol makes the membrane more fluid by preventing close packing of phospholipids.
• Plants and bacteria do not have cholesterol.
• The 4 rings are added together as rigid structure and there is a nonpolar hydrocarbon tail.
• Cholesterol sits between phospholipids (PLs).
• Cholesterol increases fluidity in the middle of the membrane due to its flexible tail.
• It decreases fluidity at the edge of the membrane due to its steroid ring.

Ethanol and Membrane Fluidity

• Ethanol increases membrane fluidity.
• One of the effects of alcohol is due to an increase in membrane fluidity.
• Chronic alcoholics compensate by increasing cholesterol content of the membrane.

Lipid Bilayer

• Lipid bilayers are asymmetric.
• The two layers have different lipid compositions.
• Glycolipids are found only on the outside.
• Transverse diffusion (flip-flop) occurs about once every 3 days,
• Phospholipid translocators, known as flippases, catalyze the flip-flop event to maintain phospholipids in the correct monolayer.

Membrane Synthesis

• Endoplasmic reticulum (ER) is critical to membrane synthesis.
• Smooth ER is lipid biosynthesis.
• Proteins destined for membranes are synthesized on one membrane of the endoplasmic reticulum (not in the ER)
• The membrane's synthesis and modification by the ER determines the asymmetric distribution of lipids, proteins, and carbohydrates.

Membrane Proteins

• Membrane proteins have a clear direction and cannot flip flop
• Carbohydrates, when present, are restricted to the cell's exterior.
• Membrane proteins have many functions.
• They includes transporters, receptors, enzymes and in anchoring molecules.

Types of Membrane Proteins

• Integral membrane proteins traverse the membrane and are also known as transmembrane proteins.
• Peripheral membrane proteins associate with one face of the membrane and are cytosolic or exoplasmic
• Some proteins bind to the surface of integral proteins.
• Some are covalently anchored to the membrane by modification.
• Proteins that traverse the membrane at least once are types of integral membrane proteins.
• Single span hydrophobic a-helix are type of integral membrane protein, where C- or N-terminal can be intra-cellular
• Multi-spanning containing integral proteins are another type of integral membrane protein and have 7 transmembrane helix.
• B-barrel protein forming a pore.

Membrane Topology

• Membrane topology is the arrangement relative to the membrane and it does not change.
• Topology is maintained by hydrophobic and electrostatic interactions.
• Positively charged AAs interact with negatively charged lipid head groups.
• Helices are hydrophobic where they interact with membrane.
• Hydrophobic where they interact with membrane hydrophilic environment in middle of pore.
• Extra and intra-cellular environments differ such as glycosylations and S-S bonds
• Loops can form binding sites
• There may be entire protein domains either extra- or intra cellularly.

Single Spanning Proteins

• Single spanning membrane proteins use a hydrophobic a-helix rather than a hydrophobic ẞ-strand.
• The amino acids can't hide the backbone and can't have H-bonding happen inside the membrane because of a hydrophobic environment

ICAM

• ICAM has several extracellular domains.
• ICAM is involved in cell-cell adhesion
• It is expressed on cells of the immune system and endothelial cells and is upregulated during inflammation
• The protein has 5 extracellular immunoglobulin domains, single transmembrane spanning helix, and short cytoplasmic tail that is extremely stable and versatile.
• Domain types get used repeatedly in different proteins and adapts to its function as a neutrophil.

Bacteriorhodopsin

• Light causes conformational change in retinal.
• Protons are then pumped from the cytosol to the extracellular space.
• The proton gradient is used for photosynthesis.

Hydrophobicity

• Hydrophobicity can be used to predict secondary structure via hydropathy index.
• Where the transmembrane helices are.

Porins

• Porins form a barrel shaped structure with a pore in the center.
• They have a hydrophobic exterior and a hydrophilic interior.
• Has 8 ẞ-strands
• Is are sucrose transporter.
• Allows bacteria to take up sucrose (disaccharide)
• The barrel has 18 ẞ-strands
• The outer membrane protein A is a membrane part of OmpA
• It is a multifunctional E. coli protein with functions in adhesion, invasion and biofilm formation.

Peripheral Membrane Proteins

• Peripheral membrane proteins do not interact with the hydrophobic core of the membrane
• They can be cytoplasmic or exoplasmic and do not change their orientation
• They interact with lipid head groups and integral membrane proteins.
• Interactions are non-covalent, but instead utilize electrostatic interactions, Hydrogen bonds, and van der Waals interactions.
• Palmitylation is one of many types of lipid anchors and is involved in signal transduction.

Proteins

• Proteins are anchored to the membrane through hydrocarbon groups.
• The protein is covalently attached to a hydrocarbon group via a post-translational modification.
• Hydrophobic hydrocarbon group inserts into the lipid bilayer

Ankyrin and Spectrin

• Spectrin is a cytoskeleton protein creating a scaffold on the intra-cellular side of membrane.
• Ankyrin binds to several integral membrane proteins and to spectrin to maintain plasma membrane integrity via the spectrin-actin based cytoskeletal structure.

Carbohydrates

• Cells are covered in carbohydrates on the exoplasmic side of membranes.
• They are attached to both lipids (glycolipids) and proteins (glycoproteins).
• The glycocalyx is a network of glycoproteins with mucus-like consistency that is hydrophilic and acts as a slime on the extracellular.
• Roles include: Physical barrier that protects against viruses and bacteria, Mechanosensing, Possible roles in cell shape.

Structures

• Most proteins have at least one carbohydrate unit.
• Few lipids have carbohydrate units.
• They exist as either oligosaccharide chains or single sugar residues.
• Glycoproteins usually have oligosaccharide chains and glycolipids usually have single sugar residues.
• Cell-cell recognition, communication and adhesion
• Important in the function of the immune system.
• It allows for distinguishing self and non-self in infection and transplantation.

Proteins and Cell Recognition

• Sugar binding proteins (lectins) bind bacterial surface sugars.
• They recognize cell-cell recognition, communication and adhesion.
• This is especially important in immune responses for distinguishing self and non-self.
• Examples of carbohydrates are present on different blood group antigens.
• Genes determine the enzymes and blood group type.

Key Messages

• Membrane fluidity is crucial for correct functioning of cells
• Membrane proteins have a variety of different structures and can be integral or peripheral.
• Glycans are on the outside of membranes and attached to either proteins or lipids.