Plasma Membrane Proteins

Plasma Membrane Proteins

Which component helps in proper folding of β-barrel membrane proteins?

Biogenesis of beta-barrel outer membrane proteins requires specialized folding and insertion systems preventing aggregation of beta-strands rich in hydrophobic residues. After Sec dependent translocation across inner membrane nascent unfolded chain enters periplasm where holdase chaperones SurA peptidyl prolyl isomerase and Skp trimeric cavity chaperone maintain unfolded state, DegP protease quality control degrades misfolded species. Targeting to BAM beta-barrel assembly machinery comprising central BamA sixteen-strand beta-barrel itself with five N-terminal POTRA domains that bind substrates

Ref: Wimley, The Versatile Beta-Barrel Membrane Protein Folding, Curr Opin Struct Biol 2003.

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Plasma Membrane Proteins

Which of the following statements about membrane proteins is FALSE?

Fluid mosaic model postulates membrane proteins diffuse laterally, associate with cytoskeleton, function as enzymes receptors carriers and structural anchors. Transverse movement of proteins between leaflets termed flip-flop faces enormous energetic barrier because large hydrophilic extramembranous domains containing charged residues lysine arginine aspartate glutamate would need dehydration and passage through hydrocarbon core twenty to thirty angstroms low dielectric constant incurring free energy tens kilocalories plus disruption of established orientation determined during biogenesis at Se

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, Membrane Protein Topology and Asymmetry.

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Plasma Membrane Proteins

Which of the following proteins uses a GPI anchor to attach to the membrane?

Classification of lipid-anchored proteins includes GPI anchor addition in endoplasmic reticulum lumen via multi-step pathway conserved across eukaryotes. GPI precursor synthesis starts on cytoplasmic face adding N-acetylglucosamine to phosphatidylinositol by PIG-A, deacetylation by PIG-L, flipping to luminal side by flippase, addition of three mannose residues from dolichol-phosphate-mannose by PIG-M PIG-V PIG-B, addition of phosphoethanolamine by PIG-O PIG-F, final transfer en bloc to C-terminal GPI signal peptide cleavage site containing omega site small residues by GPI transamidase comprisi

Ref: Mayor and Riezman, Sorting GPI-Anchored Proteins, Nature Reviews Mol Cell Biol 2004.

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Plasma Membrane Proteins

Which RBC membrane protein is essential for Cl⁻ transport?

Physiology of carbon dioxide delivery relies on chloride bicarbonate exchange across erythrocyte membrane rather than G protein signaling. Authentic transporter Band 3 anion exchanger 1 SLC4A1 abundant integral protein fourteen transmembrane helices catalyzing electroneutral one to one antiport of chloride for bicarbonate at high turnover supporting conversion of CO2 to bicarbonate via carbonic anhydrase for plasma carriage known as chloride shift Hamburger phenomenon. At pulmonary capillaries exchange reverses enabling CO2 exhalation. GPCR family members are seven-transmembrane receptors coup

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 15, AE1 Band 3 Chloride-Bicarbonate Exchanger.

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Plasma Membrane Proteins

Which of the following properties affects membrane protein mobility?

Diffusion behavior of membrane proteins governed by hydrodynamic and cytoskeletal factors. Saffman-Delbruck model relates diffusion coefficient D to membrane thickness h viscosity mu m and protein radius r through D proportional to natural log of membrane to protein size. Number of transmembrane domains increases effective radius and frictional drag because each helix engages annular lipids diffusing as complex and more extensive hydrophobic surface contacts viscous acyl chains. Single-pass glycophorin diffusion coefficient about five times ten to minus nine centimeters squared per second fast

Ref: Kusumi et al., Paradigm Shift in Membrane Protein Diffusion and Corral Model, Annu Rev Biophys.

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Plasma Membrane Proteins

Which of the following is a common feature of transmembrane β-barrel proteins?

Beta-barrel membrane proteins display characteristic functional and structural attributes correlating with aqueous pore formation. Architecture comprises even number amphipathic beta-strands eight to twenty four organized antiparallel hydrogen bonded sheet closed cylindrically first strand bonded last strand strands tilted thirty to sixty degrees relative to barrel axis. Even residues hydrophobic facing lipid core, odd hydrophilic facing lumen generating water-filled channel diameter about seven to fifteen angstroms variable. Extracellular loops often long folding into pore constriction loop t

Ref: Nikaido, Molecular Basis of Bacterial Outer Membrane Permeability, Microbiol Mol Biol Rev 2003.

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Plasma Membrane Proteins

Which technique can be used to determine if a membrane protein is inside-out or right-side-out?

Determining orientation of membrane proteins after reconstitution into artificial vesicles essential for establishing vectorial transport competence. Proteoliposomes generated by removing detergent via dialysis may incorporate proteins randomly right-side-out exposing original extracellular domains outward or inside-out exposing cytoplasmic domains outward leading to mixed activity. Sidedness assay exploits membrane impermeability of antibodies one hundred fifty kilodaltons IgG unable to cross sealed bilayer. Intact vesicles incubated with monoclonal antibody against known extracellular epitop

Ref: Rohde et al., Determination of Membrane Protein Orientation by Antibody Labeling, J Mol Biol Methods.

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Plasma Membrane Proteins

Which protein is responsible for maintaining the biconcave shape of RBCs?

Maintenance of biconcave disc shape maximizing surface to volume ratio for deformability and gas exchange depends on membrane skeleton elasticity and organization. Spectrin consists of alpha two hundred eighty kilodalton and beta two hundred forty six kilodalton subunits each containing twenty triple-helical spectrin repeats of one hundred six residues forming elongated flexible anti-parallel heterodimer one hundred nanometers long, two heterodimers associate head to head via helical bundle forming tetramer about two hundred nanometers acting as entropic spring with persistence length about te

Ref: Lux and Palek, Erythrocyte Membrane Skeleton and Spectrin Elasticity, Blood Cells.

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Plasma Membrane Proteins

Which of the following sequences is highly conserved in Aquaporins?

Highly conserved sequence motif in aquaporin family defines aqueous pore architecture and selectivity. Loop B half-helix and Loop E half-helix each dip into membrane from opposite sides each containing invariant tripeptide Asn-Pro-Ala forming two NPA boxes meeting at center of channel creating electrostatic barrier and orienting water molecules in opposite NPA asparagine carbonyl hydrogen bond donors. Proline introduces kink allowing asparagine side chain to project into pore, alanine stabilizes packing between half-helix and transmembrane helices. Structural alignments show two NPA motifs gen

Ref: Fujiyoshi et al., Structure of Aquaporin Water Channel and NPA Motif, Nature 2002.

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Plasma Membrane Proteins

Which statement about Glycophorin A is correct?

Detailed topology of glycophorin A major human erythrocyte sialoglycoprotein clarifies single-pass characteristics. Gene GYPA on chromosome four encodes one hundred fifty residues including cleavable signal peptide. Mature protein seventy kilodalton apparent due to extensive glycosylation: seventy residue extracellular domain heavily O-glycosylated fifteen O-linked tetrasaccharides NeuAc alpha2-3 Gal beta1-3 GalNAc plus one N-linked complex chain adding sialic acid dense negative charge defining MNS blood group M and N antigens and preventing rouleaux formation. Hydrophobic anchor residues sev

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, Glycophorin A Structure and Topology.

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Plasma Membrane Proteins

Which of the following describes the function of hydrophobic photoaffinity labeling?

Mapping membrane-embedded regions benefits from probes restricted to hydrophobic phase. Hydrophobic photoaffinity labeling utilizes synthetic phospholipid analogues bearing photoactivatable moieties including aryl azide generating nitrene upon UV about three hundred fifty nanometers, benzophenone generating ketyl radical, diazirine generating carbene highly reactive inserting into C-H bonds within three to five angstroms. Probes partition spontaneously into acyl chain region due to hydrophobicity, concentration in bilayer interior high. Upon irradiation during brief pulse, covalent adducts for

Ref: Brunner, Photolabeling and Hydrophobic Core Mapping, Annu Rev Biochem 1993.

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Plasma Membrane Proteins

Which of the following molecules can be transported by Aquaporin?

Selectivity of aquaporin pores illustrates precise molecular sieving. Aquaporin-1 monomer six helical bundle creates hourglass pore length approximately twenty angstroms with two constrictions: extracellular aromatic arginine filter formed by Arg195 guanidinium, His180 imidazole, Phe56 phenyl providing size filter diameter two point eight angstroms allowing water kinetic diameter two point zero angstroms but excluding hydrated sodium diameter seven point two angstroms, glucose eight angstroms, urea larger, and electrostatic barrier preventing proton conductance via positive Arg repulsion break

Ref: Verkman et al., Aquaporin Water Channels – Physiology and Selectivity, Nature Reviews Mol Cell Biol.

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