Lipid Link Protein

Lipid Link Protein

Which of the following lipids forms a thioester bond with cysteine?

Distinction between cysteine lipidations clarifies regulation of membrane affinity. S-palmitoylation involves nucleophilic attack of cysteine thiolate on palmitoyl-CoA thioester producing thioester bond between sixteen-carbon saturated palmitate and side chain catalyzed by membrane-integrated DHHC zinc finger enzymes containing conserved Asp-His-His-Cys motif utilizing palmitoyl-CoA donor. This thioester is reversible rapidly cleaved by serine hydrolases APT1, APT2, PPT1 and ABHD17 allowing depalmitoylation cycles controlling localization of HRas trafficking between Golgi and plasma membrane,

Ref: Resh, Palmitoylation of Ligands, Receptors and Intracellular Signaling Molecules, Sci STKE 2006.

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Lipid Link Protein

Which is not a function of lipid-linked proteins?

Functional spectrum of lipid-linked proteins encompasses dynamic localization and signal integration far beyond simple tethering. N-myristoylated and palmitoylated kinases like Src family and non-receptor tyrosine kinases concentrate at inner leaflet raft domains upon growth factor stimulation scaffolding MAPK, PI3K pathways regulating proliferation and cytoskeletal remodeling. Small GTPases Ras Rab and Arf require prenylation plus secondary palmitoylation for vesicular transport from endoplasmic reticulum through Golgi to plasma membrane controlling vesicle budding and fusion. GPI anchors of

Ref: Bhatnagar and Gordon, Understanding Protein Lipidation, J Cell Biol Reviews.

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Lipid Link Protein

What type of lipid linkage does CD59 have?

CD59 protectin MIRL MAC-inhibitory protein exemplifies complement regulation through GPI anchoring. Gene located chromosome eleven encodes one hundred twenty eight amino acids including twenty six residue C-terminal GPI signal peptide removed in endoplasmic reticulum. Preassembled GPI precursor synthesized via stepwise addition of N-acetylglucosamine to phosphatidylinositol, deacetylation, addition of three mannose residues via PIG enzymes, and phosphoethanolamine addition is transferred en bloc by GPI transamidase containing PIG-K catalytic subunit forming amide bond between protein and phosp

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Section: GPI-Anchored Proteins – CD59 and Complement Regulation.

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Lipid Link Protein

What happens when detergent concentration is below the CMC?

Understanding CMC highlights phase behavior of amphiphiles. At subsaturation detergent monomers exist dissolved in aqueous phase hydration shell disordered, partial insertion into outer leaflet may increase permeability but leaves bilayer structure largely intact. When monomer concentration reaches thermodynamic threshold where entropic cost of ordering water around hydrophobic tails outweighs micellization free energy, spontaneous aggregation into micelles occurs with hydrocarbon tails sequestered interior and polar heads facing water forming spherical or ellipsoidal particles thirty to fifty

Ref: le Maire et al., Interaction of Membrane Proteins and Detergents, Biochim Biophys Acta 2000.

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Lipid Link Protein

Which of the following is a membrane-permeable detergent?

Detergent physicochemical properties determine permeabilization without denaturation. Triton X-100 tert-octylphenyl polyethoxyethanol consists of hydrophobic tert-octylphenol moiety and hydrophilic polyoxyethylene chain averaging nine to ten units, conferring CMC near zero point two four millimolar, aggregation number about one hundred to one fifty and micellar weight around ninety kilodalton. It partitions efficiently into phospholipid bilayers increasing permeability to macromolecules like antibodies and substrates, forming mixed micelles that solubilize lipids while protein-protein electros

Ref: Seddon et al., Methods Mol Biol, Membrane Protein Solubilization and Detergent Properties.

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Lipid Link Protein

Why does palmitoylation enhance membrane association?

Increasing membrane residence time via acylation leverages hydrophobic insertion energy of long chain fatty acid. S-palmitoylation attaches sixteen-carbon saturated palmitate from palmitoyl-CoA donor to cysteine thiol via thioester bond catalyzed by twenty three mammalian Asp-His-His-Cys palmitoyl acyltransferases resident in endoplasmic reticulum, Golgi and plasma membrane, reversible by cytosolic acyl-protein thioesterases APT1, APT2 and ABHD17 family enabling dynamic cycling within minutes to hours responsive to neuronal activity and growth factor signaling. Added acyl chain raises partitio

Ref: Linder and Deschenes, Protein Palmitoylation and Membrane Targeting, Nature Reviews Mol Cell Biol 2007.

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Lipid Link Protein

What happens if a non-ionic detergent like Triton X-100 is used for extraction?

Successful extraction of functional membrane proteins relies on preserving native fold during bilayer dissolution. Non-ionic detergents like Triton X-100, n-octyl beta-D-glucoside and n-dodecyl beta-D-maltoside possess uncharged polar heads composed of polyoxyethylene chains or sugar residues. Their aliphatic tails insert between phospholipid acyl chains disrupting lipid-lipid and lipid-protein contacts while headgroups remain non-interacting with polypeptide backbone. Mixed micelles of approximately three to five nanometers surround hydrophobic transmembrane regions, extramembranous loops rem

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, Non-Ionic vs Ionic Detergent Extraction.

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Lipid Link Protein

What is the minimum amount of detergent needed for effective solubilization if the CMC is 5 mM?

Solubilization efficiency depends on detergent self-assembly thermodynamics above critical micelle concentration. Below CMC monomers dissolve individually with limited bilayer insertion causing minor perturbation without micelle formation. Above CMC cooperative aggregation produces spherical micelles of thirty to hundred monomers with hydrophobic core shielded and hydrophilic exterior, light scattering increases sharply and free monomer concentration remains near CMC. Micelles provide hydrophobic compartments that encapsulate lipids and transmembrane segments into water-soluble mixed micelles

Ref: Rosen and DeLisi, Detergent Properties and CMC in Membrane Protein Solubilization, Methods Enzymol.

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Lipid Link Protein

Which of the following proteins is typically not lipid-anchored?

Membrane protein classification by lipid modification clarifies attachment without transmembrane span. Ras GTPases undergo prenylation by farnesyltransferase recognizing C-terminal CAAX motif, subsequent RCE1 proteolysis, ICMT methylation, and for N-Ras and H-Ras additional Golgi palmitoylation by DHHC enzymes driving plasma membrane targeting and partitioning into ordered domains for MAPK signaling and transformation. Src family kinases c-Src, Fyn, Lck, Yes undergo cotranslational N-myristoylation at glycine two plus reversible S-palmitoylation for stable inner leaflet association recruiting

Ref: Pollard and Earnshaw, Cell Biology, 3rd ed., Chapter 17, Actin Filaments and Lipid-Anchored Proteins.

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Lipid Link Protein

What is the length of a single transmembrane α-helix in lipid bilayers if it spans ~3 nm?

Geometrical constraints of fluid phospholipid bilayer dictate minimal span length for alpha-helical membrane proteins. Hydrophobic thickness between carbonyl regions of liquid-crystalline phosphatidylcholine bilayer averages three point zero nanometers. An alpha helix advances zero point fifteen nanometers per residue along axis with three point six residues per turn, pitch zero point five four nanometers stabilized by intramolecular hydrogen bonds between carbonyl i and amide i plus four. Dividing three nanometers by zero point fifteen per residue yields about twenty residues required for per

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, Transmembrane Alpha-Helix Length and Bilayer Thickness.

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Lipid Link Protein

What type of bond formation occurs in myristoylation?

Myristoylation represents cotranslational irreversible lipidation catalyzed by cytosolic N-myristoyltransferases NMT1 and NMT2. After methionine aminopeptidase removes initiator methionine, glycine at position two becomes exposed as acceptor. NMT binds myristoyl-CoA, a fourteen-carbon saturated fatty acyl thioester, and transfers acyl chain to alpha-amino group of N-terminal glycine forming stable amide linkage resistant to neutral hydroxylamine, alkaline treatment, and reducing agents, unlike labile thioesters. The myristate provides approximately eight kilocalories per mol hydrophobic insert

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10, Protein Lipidation and Myristoylation.

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Lipid Link Protein

Which technique is used to determine whether a protein is GPI-anchored?

Distinguishing GPI anchoring from transmembrane helices or other lipid modifications requires an enzymatic strategy that specifically targets the glycolipid linkage chemistry. A mature GPI anchor contains phosphoethanolamine in amide bond to the protein C-terminus, a conserved tetrasaccharide core of three mannose residues and glucosamine, and phosphatidylinositol whose diacylglycerol or alkylacyl side chains embed in the exoplasmic leaflet. Bacterial phosphatidylinositol-specific phospholipase C from Bacillus thuringiensis cleaves the phosphodiester between glycerol and phosphate, releasing p

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, GPI-anchored proteins and PI-PLC assay.

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