Membrane Proteins Basic Overview

Core Components and Properties of Biological Membranes:

Biological membranes are not static barriers but dynamic, fluid, and heterogeneous structures. Their composition directly supports the function of the proteins embedded within them.

  • Phospholipids (Glycerophospholipids): Built on a glycerol backbone, these are amphipathic (possessing both hydrophobic tails and hydrophilic head groups).

    • Variation: Diversity arises from head group changes (e.g., anionic Phosphatidylserine (PS) or zwitterionic Phosphatidylethanolamine (PE)) and fatty acid chain length/saturation.

    • Fluidity: Saturation dictates rigidity. Bacteria regulate membrane fluidity by increasing unsaturated lipids at lower temperatures and saturated lipids at higher temperatures.

  • Sphingolipids: Based on a sphingosine molecule. Examples include sphingomyelin (common in nerve cells) and glycolipids (sugar-conjugated lipids).

  • Sterols: Eukaryotes use cholesterol to maintain fluidity by preventing tight phospholipid packing. Prokaryotes lack sterols; fungi use ergosterols, and plants use cytosterols.

  • The Fluid Mosaic Model: Proposed by Singer and Nicholson (1972), it describes proteins "floating like icebergs" in a sea of lipids.

    • Evidence: Freeze fracture electron microscopy visualised protein nodules within the bilayer. FRAP (Fluorescence Recovery After Photobleaching) proved that lipids and proteins move laterally within the membrane.

Classification and Functions of Membrane Proteins:

Approximately 25% of all proteins are membrane-associated, and they represent 50% of all drug targets.

Structural Classifications:

  • Integral (Transmembrane): Span the entire bilayer. They have hydrophobic surfaces (Leucine, Phenylalanine, etc.) to interact with lipid tails.

  • Peripheral: Associated with the surface through interactions with integral proteins or lipid head groups. They behave like water-soluble proteins if removed.

  • Anchored: Water-soluble proteins attached to the membrane via a covalently linked lipid "anchor".

Functional Classifications:

  • Transporters: Regulate the influx and efflux of polar substances (ions, sugars, etc.) across the impermeable bilayer. Example: GLUT4 for glucose.

  • Receptors: Conduct signals across the membrane without the ligand entering the cell. Example: Insulin receptor or Rhodopsin (light sensing).

  • Ion Channels: Facilitate rapid movement of ions down electrochemical gradients. Unlike transporters, channels form a continuous pore and do not require a conformational change for every single molecule.

  • Enzymes: Facilitate chemical reactions. Example: Proteins in the electron transport chain (mitochondria) or thylakoid membranes.

  • Recognition/Markers: Sites for viral/bacterial docking (e.g., CCR5 for HIV) or immune flags (e.g., MHC complex presenting peptides to T-cells).

Secondary Structure and Topology Prediction:

Membrane proteins must "hide" the polar backbone of their peptide bonds (NH and CO groups) to remain stable within the hydrophobic lipid environment.

Structural Motifs:

  • Alpha Helices: The most common motif. A 3.6-residue-per-turn helix hides polar groups in the centre. Approximately 25 amino acids are required to span a 3-4 nm membrane.

  • Beta Barrels (Beta Sheets): Found exclusively in the outer membranes of bacteria and eukaryotic organelles (mitochondria/chloroplasts). They form porins with a hydrophilic interior and hydrophobic exterior.

Topology Prediction:

  • Hydrophobicity Plots: Predict membrane-spanning regions by scanning protein sequences for windows of ~20-25 hydrophobic residues.

  • Refinement: Accuracy is improved by accounting for "capping" residues (e.g., aromatic residues like Tryptophan) that interact with lipid head groups or cholesterol.

The Linear Process of Purification and Reconstitution:

Studying membrane proteins requires extracting them from the lipid environment while maintaining their fold.

  • Expression: Producing sufficient protein for study (often discussed in practicals).

  • Isolation: Separating the cell membrane from the cytoplasm.

    • Lysis: Cells are broken via sonication (high-energy sound) or pressure-based methods (French press).

    • Centrifugation: Low-speed (3,000 G) removes debris/unbroken cells. Ultracentrifugation (100,000 G) pellets the purified cell membranes.

  • Solubilization: Using detergents to remove lipids and make the protein water-soluble.

    • Ionic Detergents (e.g., SDS): Harsh; disrupt protein folding. Rarely used for functional studies.

    • Non-ionic Detergents (e.g., Triton X-100): Mild; break lipid-protein bonds without denaturing the protein.

    • Zwitterionic Detergents (e.g., CHAPS): Useful for breaking protein-protein complexes.

    • Mechanism: Detergents form mixed micelles with proteins once the Critical Micelle Concentration (CMC) is reached.

  • Purification: Using standard chromatography (Affinity, Ion Exchange, Size Exclusion) while keeping detergent present to prevent the protein from precipitating.

  • Reconstitution: Returning the pure protein to a lipid bilayer to restore activity.

    • Methods: Detergent removal via dialysis, dilution, or using polystyrene Bio-Beads which specifically bind detergents but not lipids