Detailed Notes on Membranes and Membrane Proteins

Membrane: Their Structure, Function, and Chemistry

7.4 Membrane Proteins: The “Mosaic” Part of the Model

  • The mosaic part of the fluid mosaic model consists of lipid rafts and other lipid domains.
  • However, membrane proteins are the main components of this model, indicating their vital role in the structure and function of membranes.

Evidence Supporting the Fluid Mosaic Model

Freeze-Fracture Microscopy
  • Support for the fluid mosaic model emerged from studies that utilized freeze fracturing technique.
  • This method involves:
    • Freezing a bilayer or membrane and then sharply cutting it with a diamond knife.
    • The fracture usually follows the plane between the two lipid layers of the membrane, revealing the structure of the membrane.
  • When a fracture plane separates the monolayers of the membrane, the hydrophobic interior is exposed, allowing visualization of the inner surfaces of both monolayers.
Observation of Membrane Faces
  • Integral membrane proteins that remain associated with the outer monolayer appear on the E (exoplasmic) face, whereas those associated with the inner monolayer are seen on the P (protoplasmic) face.
  • Electron micrographs taken from freeze-fractured membranes show that individual proteins appear as small particles embedded in either face of the membrane.

Types of Membrane Proteins

Classification of Membrane Proteins
  • Membrane proteins can be categorized based on their hydrophobicity and localization:
    • Integral Membrane Proteins: These proteins are embedded within the lipid bilayer due to their hydrophobic regions.
    • Peripheral Membrane Proteins: These proteins are hydrophilic and located on the surface of the bilayer without penetrating it.
    • Lipid-Anchored Proteins: These proteins are also hydrophilic but are covalently linked to lipid molecules within the bilayer, anchoring them to the membrane.

Characteristics of Membrane Proteins

Integral Membrane Proteins
  • Integral membrane proteins consist of hydrophobic regions that are located within the lipid bilayer, making them difficult to remove using standard isolation techniques.
  • Some integral membrane proteins are classified as:
    • Integral Monotropic Proteins: Embedded in only one side of the bilayer.
    • Transmembrane Proteins: Span the membrane and protrude on both sides, classified as:
    • Singlepass Proteins: Cross the membrane once.
    • Multipass Proteins: Cross the membrane multiple times (2–20 or more segments).
Structure of Transmembrane Proteins
  • Transmembrane proteins are anchored to the lipid bilayer by hydrophobic transmembrane segments.
  • Typically, the polypeptide chain spans the membrane in an α-helical conformation, approximately 20–30 amino acids long.
  • Some transmembrane proteins may also be arranged in a closed β sheet structure referred to as a β barrel.
Singlepass Membrane Proteins
  • Singlepass membrane proteins feature a C-terminus extending from one surface of the membrane and an N-terminus extending from the opposite surface.
  • Example: Glycophorin, a singlepass protein located in the erythrocyte plasma membrane, where the C-terminus is oriented toward the inner surface and the N-terminus extends outward.
Multipass Membrane Proteins
  • Multipass membrane proteins contain multiple transmembrane segments; for instance, bacteriorhodopsin is an example with seven transmembrane segments that form a channel.
Peripheral Membrane Proteins
  • Peripheral membrane proteins are defined by their lack of distinct hydrophobic regions, thus not penetrating the lipid bilayer.
  • These proteins bind to the surface of membranes via weak electrostatic forces and hydrogen bonds, with some hydrophobic residues aiding in anchoring them to the membrane surface.
Lipid-Anchored Membrane Proteins
  • These proteins are located on the membrane surfaces and are covalently bound to lipid molecules embedded in the bilayer.
  • Proteins on the inner surface of the plasma membrane can be linked to fatty acids or isoprenyl groups to maintain their association with the membrane.

Techniques for Analyzing Membrane Protein Structure

X-ray Crystallography
  • X-ray crystallography is a prevalent technique for determining the three-dimensional structure of proteins.
  • Integral membrane proteins pose challenges for isolation and crystallization due to their hydrophobic nature.
Protein Purification for Crystallography
  • Protein purification methods are critical for obtaining a pure protein sample necessary for crystallography:
    • Affinity Chromatography: Utilizes specific binding interactions for isolating the target proteins.
    • Ion-Exchange Chromatography: Separates proteins based on their charge.
    • Size Exclusion Chromatography: Separates proteins according to size.
    • Hydrophobic Charge Induction Chromatography (HCIC): Uses pH-dependent hydrophobic interactions to purify antibodies.
Hydropathy Analysis
  • Hydropathy analysis helps predict transmembrane segments' number and location by analyzing the protein sequence.
  • A computer program generates a hydropathy plot that identifies clusters of hydrophobic residues, providing a hydropathy index for segments of the protein.

Advances in Membrane Protein Modeling

AlphaFold
  • The AI model AlphaFold has transformed protein structure prediction.
  • Developed by John Jumper and Demis Hassabis at Google DeepMind, it plays a significant role in drug discovery by modeling protein interactions.

Functions of Membrane Proteins

Enzymatic Functions
  • Certain membrane proteins function as enzymes, localizing specific enzymatic activities to distinct membranes.
  • Examples include electron transport proteins like cytochromes and iron-sulfur proteins.
Solute Transport Across Membranes
  • Membrane transport proteins aid in nutrient movement across cellular membranes:
    • Channel Proteins: Provide hydrophilic passageways through hydrophobic membrane layers.
    • Transport ATPases: Utilize ATP energy to transport ions across membranes.
Signaling Across Membranes
  • Membrane receptors understand and mediate chemical signals impacting the cell, such as hormones or neurotransmitters.
Intercellular Communication
  • Membrane proteins are crucial for cell-to-cell communication:
    • Connexons: Form gap junctions in animal cells, enabling direct communication.
    • Plasmodesmata: Structures that facilitate communication in plant cells.
Other Cell Functions
  • Membrane proteins are involved in various cellular processes:
    • Uptake and secretion via endocytosis and exocytosis.
    • In protein targeting, sorting, and modification in the endoplasmic reticulum and Golgi complex.
    • Participation in autophagy, which is the breakdown of a cell’s own organelles.
Structural Roles of Membrane Proteins
  • Membrane-associated proteins contribute to maintaining the structural integrity, stability, and shape of the cell membrane.
    • In animal cells, a cytoskeletal meshwork of peripheral membrane proteins often lies beneath the plasma membrane to provide additional support.
Asymmetric Orientation of Membrane Proteins
  • Membrane proteins show asymmetric orientation relative to the lipid bilayer.
  • Once established in one monolayer, proteins do not typically migrate across the membrane, and all molecules of a specific protein exhibit the same orientation.
Glycosylation of Membrane Proteins
  • Glycoproteins are membrane proteins that have carbohydrate chains covalently attached to amino acid side chains, a process termed glycosylation, which occurs in the ER and Golgi.
N-linked Glycosylation
  • Involves binding oligosaccharides N-acetylglucosamine (GlcNAc) to aspartate residues within the ER.
  • This results in three main N-glycan subtypes based on side chain branching: complex, mixed, and high mannose polysaccharides.
  • The core structure for N-glycans is (asn-GlcNAc2Man3) and may be extended with terminal glycan residues consisting mainly of GlcNAc, mannose, galactose, fucose, and sialic acid (N-acetylneuraminic acid, Neu5Ac).
O-linked Glycosylation
  • O-glycosylation normally involves an oxygen bond of glycans to serine or threonine residues, usually requiring gradual monosaccharide addition.
  • This modification primarily occurs in the nucleus and cytoplasm rather than in the ER/Golgi.
  • The most common types are O-acetylgalactosamine (O-GalNAc) and O-linked-β-D-N-acetylglucosamine (O-GlcNAc).
Glycocalyx: The Highly Charged Cloak
  • Glycocalyx refers to a fibrous, highly hydrated, and charged meshwork of carbohydrates covering the cellular membrane.
  • Composed of proteoglycans and glycoproteins, glycocalyx serves multiple functions:
    • Facilitates cell attachment and clusters of growth factors.
    • Regulates access to membrane receptors and ECM remodeling.
    • Affects membrane curvature and mechanosensing, aiding cell interactions.
Membrane Protein Mobility and Anchoring
  • Membrane proteins often have restricted mobility due to anchoring to cellular structures (either internal or external).
  • Example: proteins may be anchored to cytoskeletal structures or extracellular components, influencing their positioning and function in live cell membranes, including GPI-anchored proteins, which do not reside in organized domains within live cells.