Lecture 3-2: Membrane Proteins

Membrane Proteins Overview

• Proton-driven Rotors

  • Spinach chloroplast FoF1–ATP synthase.

• Sodium-driven Rotors

  • Ilyobacter tartaricus FoF1–ATP synthase.

• Photosynthetic Membranes

  • High-light-adapted membranes from Rsp. Photometricum.

• Pore Complexes

  • Perfringolysin O (PFO).

• Oligomeric State

  • Bovine rhodopsin in native disc membranes.

• Light-Harvesting Complex

  • Structural organization of light-harvesting complex I from Rsp. Rubrum.

• Extracellular Surface

  • Gap junction hemichannels from rat liver cells recorded at pH 7.6.

Functions of Membrane Proteins

• Movement Facilitation

  • Aid transport of polar molecules across membranes.

• Structural Organization

  • Organize cytoskeletal fibers and bind to extracellular matrix.

• Bridge Formation

  • Act as connectors between the cytoskeleton and external environment.

• Molecule Detection

  • Detect specific extracellular molecules.

• Catalysis

  • Catalyze conversion of molecules, primarily in signaling pathways.

Associations of Proteins with Membranes

• Transmembrane Proteins

  • Span entire lipid bilayer, interaction on either side of membrane.

• Hydrophobic Residue Insertion

  • May only insert into one leaflet of the bilayer.

• Covalent Links

  • Proteins can be covalently linked to membrane lipids.

• Protein Interactions

  • Separate proteins associate through interactions with transmembrane proteins.

Membrane Spanning Protein Domains

• Hydrophobic Richness

  • Rich in hydrophobic side chains allowing for membrane embedding.

• Protein Folding Dynamics

  • In contrast to typical hydrophobic folding, residues in membrane segments project outward.

• Helices and Sheets

  • Outward positioning of the R groups limits lipid-tail interactions.

Aqueous Pore Proteins

• Hydrophilic Spans

  • Certain residues permit passage of hydrophilic molecules.

• Configuration

  • β-barrels and arrangements of α-helices form polar pathways.

• Common Applications

  • Ion and water channels often utilize this arrangement.

Membrane Protein Studies

• Detergent Use

  • Mild detergents like SDS and Triton X-100 can release proteins by masking hydrophobic areas.

• 3-D Structure Determination

  • X-ray crystallography remains a primary method, though newer techniques like cryo-EM and NMR offer advanced insights.

  • Example: Bacteriorhodopsin as a proton pump highlights how light absorption drives proton transport.

Cell Cortex and Membrane Proteins

• Cytoskeleton Role

  • Network forms to support cell shape by anchoring embedded proteins.

• Example in Erythrocytes

  • Actin forms complexes with transmembrane proteins, linked by spectrin tetramers.

Protein Movement Within Plasma Membrane

• Free Movement

  • Proteins can move unrestrictedly across the plasma membrane.

• Fluorescence Experiment

  • Fluorescently labeling enables tracking of protein distribution in fused human and mouse cells.

FRAP and Protein Diffusion

• Measurement Technique

  • Fluorescence Recovery After Photobleaching (FRAP) shows diffusion rates of membrane proteins.

• Bleach Recovery

  • Time for fluorescence recovery indicates movement speed of unbleached proteins.

Patterns of Protein Movement

• Types of Movement

  • Unrestricted, confined, and tethered movements vary based on molecular constraints.

• Tracking Innovations

  • Single particle tracking provides insight into individual protein movements.

Reconstituted Bilayers and Protein Function

• Individual Protein Study

  • Insertion into artificial bilayers allows specific functional analysis without interference.

• Transport Studies

  • Liposomes are preferred for studying specific transporter nuances.

Protein Movement Restrictions

• Polarization in Cells

  • Restriction often occurs in polarized cells creating specialized membrane domains.

• Mechanisms of Restriction

  • Terrestrial interactions (intracellular, extracellular, adjacent cell interactions, diffusion barriers).

Epithelial Cell Characteristics

• Polarity and Domain Creation

  • Tight junctions separate cells into apical and basolateral regions for functional specialization.

Glycosylation in Eukaryotic Cells

• Sugar Coating

  • Key molecules undergo glycosylation on lipids and proteins.

• Proteoglycans

  • Type of glycoprotein enriched with glycosaminoglycans (GAGs).

Cell Recognition Through Sugars

• Role of Glycocalyx

  • Protects cells; partakes in recognition processes during immune responses.

• Lectins' Function

  • Bind to specific sugar patterns, facilitating immune cell activity during infections.

Key Concepts Summary

• Functions of Membrane Proteins

  • Include roles as transporters, anchors, receptors, enzymes.

• Association Types

  • Integral or peripheral, with transmembrane proteins fully traversing the membrane.

• Diffusion Limitations

  • Restricted proteins exhibit various movement patterns; tethered proteins interact closely with cell structure.

• Role of Epithelial Cells

  • Exhibit unique barriers and domains to regulate material movement.

• Glycocalyx Importance

  • Provides protection and regulatory functionality in cellular interactions.