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#protein modification

18 public questions tagged with this topic.

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

The prenylation reaction occurs at which region of the protein?

Protein prenylation attaches hydrophobic isoprenoid to facilitate membrane association and protein-protein interactions. Substrate determinant is C-terminal CAAX box where C is cysteine acceptor, A aliphatic amino acids isoleucine, leucine, valine, X residue determining enzyme specificity methionine, serine, glutamine for farnesyltransferase versus leucine for geranylgeranyltransferase I, while Rab proteins contain CC, CXC motifs recognized by geranylgeranyltransferase II. Reaction catalyzed by cytosolic heterodimeric transferases FTase, GGTase I, GGTase II using farnesyl or geranylgeranyl pyr

Ref: Zhang & Casey, Annu Rev Biochem 1996, CAAX prenylation; Wang & Casey, Nat Rev Mol Cell Biol 2016.

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Lipid Link Protein

The lipid anchor in GPI-anchored proteins is attached to which part of the protein?

Glycosylphosphatidylinositol anchoring is complex post-translational modification processed in endoplasmic reticulum for about 150 human proteins destined for extracellular leaflet. Nascent proteins contain C-terminal hydrophobic signal peptide that is recognized by transamidase complex, cleaved after a specific omega residue, and replaced en bloc with preformed GPI moiety consisting of phosphatidylinositol lipid, glucosamine, three mannoses, phosphoethanolamine and galactose modifications. Attachment occurs to new C-terminus via amide linkage between protein carboxyl group and ethanolamine ph

Ref: Fujita & Kinoshita, J Lipid Res 2012, GPI anchor biosynthesis and attachment.

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Lipid Link Protein

Which amino acid is commonly involved in myristoylation?

N-myristoylation is co-translational irreversible modification attaching 14-carbon saturated fatty acid myristate from myristoyl-CoA to target protein. N-myristoyltransferase NMT recognizes N-terminal sequence Met-Gly-X-X-X-Ser/Thr after methionine aminopeptidase removes initiator methionine, exposing glycine at position 2 for amide bond formation. The glycine requirement is absolute; replacement with other residues abolishes myristoylation. Myristate anchor provides weak membrane affinity that often cooperates with additional palmitoylation or polybasic cluster to stably tether proteins to in

Ref: Resh, Biochim Biophys Acta 1999, N-myristoylation mechanisms; Farah et al., J Biol Chem 2020, NMT specificity.

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Lipid Link Protein

Which of the following statements about lipid-linked proteins is true?

Lipid-linked proteins associate with membranes via covalent attachment of hydrophobic moieties rather than simple non-specific interactions. Several distinct modifications exist: N-myristoylation adds 14-carbon myristate to N-terminal glycine after methionine removal via amide linkage, palmitoylation S-acylates internal cysteine via thioester that is reversible, prenylation attaches 15-carbon farnesyl or 20-carbon geranylgeranyl to C-terminal cysteine in CAAX motif via thioether, and glycosylphosphatidylinositol anchor attaches to C-terminus via phosphoethanolamine bridging to glycan core term

Ref: Resh, Nat Chem Biol 2013, Lipid modifications; Fujita et al., J Lipid Res 2020, GPI anchors.

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Cell Organelles

The Golgi apparatus is involved in which type of glycosylation?

Protein glycosylation diversity arises from distinct enzyme locations. N-linked glycosylation defined by oligosaccharide attachment to asparagine within Asn-X-Ser/Thr consensus begins cotranslationally in ER where oligosaccharyltransferase transfers preassembled Glc3Man9GlcNAc2 from dolichol, then trimmed by glucosidases. Extension into complex types continues in Golgi but core attachment ER-specific. O-linked glycosylation where N-acetylgalactosamine alpha linked to serine threonine initiated by family of twenty polypeptide GalNAc transferases transferring GalNAc from UDP-GalNAc directly to p

Ref: Spiro Glycobiology; O-GalNAc transferase initiates O-glycosylation in Golgi, N-linked starts ER.

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Cell Organelles

The Golgi apparatus is responsible for:

Golgi apparatus comprises four to eight flattened cisternae forming cis entry, medial processing and trans exit network with distinct enzyme gradients maintaining glycosylation sequence. Cargo arriving via COPII vesicles from ER undergoes mannose trimming by mannosidase I in cis, addition of N-acetylglucosamine by GlcNAc transferase I and II in medial, galactose and sialic acid in trans, creating complex glycans affecting half-life and receptor binding. Phosphorylation of mannose residues marks lysosomal enzymes, sulfation of proteoglycans occurs. Trans-Golgi network functions as major sorting

Ref: Rothman Cell 1994; Golgi cisternae glycosylation phosphorylation sorting TGN clathrin AP1 GGA.

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Protein Sorting and Trafficking

Which glycosylation type involves attachment to tryptophan residues?

Among glycosylation classes, attachment via carbon-carbon bond to tryptophan indole ring is chemically distinct and relatively rare. C-mannosylation involves α-D-mannopyranose linked to carbon-2 of indole of tryptophan within consensus W-X-X-W or W-X-X-C motif, catalyzed co-translationally inside ER lumen by multi-pass membrane enzymes DPY19L1 through L4 using dolichol-phosphate-mannose as high-energy donor substrate assembled on ER membrane. Modification increases hydrophobicity locally, stabilizes beta-propeller and thrombospondin type 1 repeat folds found in properdin, F-spondin, mindin, AD

Ref: Buettner et al., Mol Cell 72: 2018, C-Mannosylation of Tryptophan by DPY19.

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Protein Sorting and Trafficking

The key difference between N-linked and O-linked glycosylation is:

Two major enzymatic glycosylation systems differ fundamentally in linkage chemistry, sugar identity at attachment point, donor substrate and cellular location. N-linked glycosylation forms amide linkage between N-acetylglucosamine GlcNAc β-linked to side chain nitrogen of asparagine within consensus Asn-X-Ser/Thr where X not proline, initiated co-translationally in ER by oligosaccharyltransferase complex using dolichol-linked precursor Glc3Man9GlcNAc2 assembled on lipid carrier. Subsequent trimming and Golgi processing generate high-mannose, hybrid and complex types influencing folding and sta

Ref: Stanley et al., Essentials of Glycobiology, 4th ed., Chapter 9: N versus O Glycosylation.

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Electrophoresis

Which protein modification slows SDS-PAGE migration but doesn't change pI?

SDS-PAGE migration primarily reflects polypeptide length due to uniform SDS charge coating, overwhelming intrinsic charge. Post-translational modifications that add neutral mass retard mobility without altering isoelectric point. N-linked and O-linked glycosylation attaches bulky hydrophilic carbohydrate chains lacking proportional SDS binding and charged groups, increasing hydrodynamic size and apparent molecular weight substantially while pI remains largely unchanged because glycans are mostly neutral. This produces slower, often smeared migration. Phosphorylation, ubiquitination and acetyla

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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