Module 2: Membrane Proteins - PDF

Slide 1: Learning Objectives describe how proteins are arranged and anchored in the bilayer describe the types of protein movement in the membrane explain how detergents solubilize membrane proteins understand how red blood cell ghosts are used to determine the positioning of membrane pro...

Slide 1: Learning Objectives describe how proteins are arranged and anchored in the bilayer describe the types of protein movement in the membrane explain how detergents solubilize membrane proteins understand how red blood cell ghosts are used to determine the positioning of membrane proteins discuss the composition and role of the cell glycocalyx describe the different functions of membrane proteins Slide 2: Transmembrane Proteins with cytosolic and non-cytosolic (extracellular) domains, separated by membrane domains membrane domains: mostly amino acids with nonpolar side chains; all peptide bonds form hydrogen bonds with one another (maximized number in an α-helix) alternatively, multiple transmembrane segments of a polypeptide chain are arranged as a β-sheet (barrel) β-barrel proteins are abundant in the outer membrane of mitochondria; some form water- filled channels Slide 3: Anchors glycosylphosphatidylinositol (GPI) anchor: a glycolipid attached to the C-terminus of a protein; the two fatty acids (tails) anchor the protein to the cell membrane phospholipase C (PLC) can cleave the phosphoglycerol bond in GPI-anchored proteins to release GPI-linked proteins from the outer cell membrane GPI-linked proteins are preferentially located in lipid rafts other anchors: fatty acid chains and prenyl groups are mostly utilized on the cytosolic face of the membrane Slide 4: Functions of Membrane Proteins junctions: to connect and join two cells enzymes: to catalyze reactions transport: for facilitated diffusion, active transport recognition: for cell surface markers ('labels') anchorage: to attach to the cytoskeleton and extracellular matrix signaling: to function as receptors (usually integral proteins) Slide 5: Membrane Protein Diffusion membrane proteins rotate about an axis perpendicular to the bilayer (rotational diffusion) or move laterally (lateral diffusion) evidence: the mixing of membrane proteins in hybrid cells from mice and humans lateral diffusion is measured with fluorescence recovery after photobleaching (FRAP) Slide 6: Fluorescence Recovery After Photobleaching (FRAP) proteins can be tethered to assemblies of molecules some membrane proteins are confined to domains (apical versus basolateral); tight junctions support the restriction neurons contain membrane domains enclosing the cell body and dendrites, and domains enclosing the axons Slide 7: How to Solubilize Membrane Proteins membrane proteins are solubilized by agents that disrupt hydrophobic associations and destroy the lipid bilayer detergents are amphipathic molecules that form micelles in water; the hydrophobic ends of detergents bind to the hydrophobic regions of the membrane proteins, displacing the lipid molecules the polar end of the detergent brings the proteins into solution as detergent-protein complexes the polar (hydrophilic) ends of detergents can be either charged (ionic), as in sodium dodecyl sulfate (SDS), or uncharged (nonionic), as in the Triton detergents Slide 8: Red Blood Cell Ghosts red blood cells - no nuclei, no internal organelles; the plasma membrane is the only membrane empty red blood cell membranes (ghosts) preparation: cells are exposed to a lower salt concentration than the cells’ interior; water flows into the cells, lyses them, and releases the cytosolic proteins the 'sidedness' of a membrane protein is determined with a label (e.g., fluorescent marker) that cannot penetrate the lipid bilayer; the marker binds the exposed membrane side of a protein; the membranes are solubilized with detergent, the proteins are separated by SDS polyacrylamide-gel electrophoresis alternatively, the external or internal surface is exposed to proteolytic enzymes that are membrane-impermeant: if a protein is partially digested from both sides, it must be a transmembrane protein Slide 9: Spectrin, Glycophorin, and Band 3 Spectrin: Peripheral membrane protein; heterodimers → tetramers that are linked to cytoskeletal proteins to form a network under the plasma membrane; the spectrin network enables the cells to withstand mechanical stress Glycophorin: single-pass transmembrane, usually a homodimer; on the external surface its hydrophilic N-terminus carries a carbohydrate Band 3: multi-pass transmembrane protein; anion transporter: HCO3- crosses the membrane in exchange for Cl-;increases the delivery of CO2 by the blood to the lungs Slide 10: Glycosylated Membrane Proteins most transmembrane proteins are glycosylated the oligosaccharides are always on the non-cytosolic (extracellular) side of the membrane some serve as receptors sulfhydryl groups in the cytosolic domain do not normally form disulfide bonds (-S-S-) because the reducing environment in the cytosol maintains these groups in their reduced (- SH) form Slide 11: Glycocalyx carbohydrate-rich zone on the cell surface (glycoproteins, glycolipids, proteoglycans) 'ID-badge' the main function is protection against mechanical and chemical damage glycoproteins: proteins with short highly branched glycan chains with no repeating unit (A) proteoglycans: type of glycoproteins with at least one covalently linked glycosaminoglycan chain (a long chain with disaccharides as repeating structures) (B) Slide 12: Putting It Together polypeptides cross the bilayer as a single α helix, as a series of α-helices, or as a β-sheet ('barrel'); some proteins are attached to either side of the membrane (non-covalently to transmembrane proteins or covalently to lipids) membrane proteins function as receptors, enzymes, transport proteins, anchors, recognition labels membrane proteins diffuse in the membrane or are immobilized on the outer lipid monolayer: proteins and lipids frequently have oligosaccharide (i.e., carbohydrate) chains the glycocalyx protects cells from mechanical and chemical damage