The covalent structure of proteins is commonly modified in structurally and functionally important ways beyond the linear coupling of amino acids via the peptide bond. Regulated proteolysis can be considered a PTM and can serve an important regulatory role, as in the cleavage of prothrombin in the blood clotting cascade.

A number of functional groups are appended to proteins to regulate their function, localization, protein interactions, and degradation. Examples of these PTMs include phosphorylation, glycosylation, ubiquitination, methylation, acetylation, and lipidation.[1] PTMs occur at distinct amino acid side chains or peptide linkages and are most often mediated by enzymatic activity and can occur at any step in the “life cycle” of a protein. As discussed later, a number of protein domains have evolved to recognize and bind specifically to proteins labeled by a particular PTM. Protein phosphorylation, most commonly on serine, threonine, or tyrosine residues, is one of the most important and well-studied PTMs. Phosphorylation is mediated by protein kinases and can activate or deactivate many enzymes through conformational changes and as such plays a critical role in the regulation of many cellular processes, including cell cycle, growth, apoptosis, and signal transduction pathways. Protein glycosylation encompasses a diverse selection of sugar-moiety additions to proteins that ranges from simple monosaccharide modifications to highly complex branched polysaccharides. Glycosylation has significant effects on protein folding, conformation, distribution, stability, and activity. Carbohydrates in the form of asparagine-linked (N-linked) or serine/threonine-linked (O-linked) oligosaccharides are major structural components of many cell surface and secreted proteins and also many viral proteins. Protein methylation on argi nine or lysine residues is carried out by methyltransferases with S-adenosyl methionine (SAM) as the primary methyl group donor. Methylation is an important mechanism of epigenetic regulation because histone methylation and demethylation influence the avail ability of DNA for transcription. N-acetylation, the transfer of an acetyl group to the amine nitrogen at the N-terminus of the polypeptide chain, occurs in a majority of eukaryotic proteins. Lysine acetylation and deacetylation is an important regulatory mechanism in a number of proteins. It is best characterized in histones, where histone acetyl transferases (HATs) and histone deacetylases (HDACs) regulate gene expression via modification of histone tails. Many cytoplasmic proteins are also acetylated, and therefore acetylation seems to play a greater role in cell biology than simply transcriptional regulation.[2] Lipidation is a modification that targets proteins to mem branes in organelles, vesicles, and the plasma membrane. Examples of lipidation include myristoylation, palmitoylation, and prenylation. Each type of modification gives proteins distinct membrane affinities, although all types of lipidation increase the hydrophobicity of a protein and thus its affinity for membranes. In N-myrisoylation, the myristoyl group (14-carbon saturated fatty acid) is transferred to an N-terminal glycine by N-myristoyltransferase. The myristoyl group does not always permanently anchor the protein in the mem brane; in a number of proteins the N-terminal myristoyl group has been observed to pack into the protein core. N-myristoylation can therefore act as a conformational localization switch in which protein conformational changes influence the availability of the handle for membrane attachment.

Finally, ubiquitination (or equivalently, ubiquitylation) involves modification of a target protein with the protein ubiquitin, which is typically attached to a lysine residue via an isopeptide bond with the C-terminus of ubiquitin. Modification with ubiquitin, either singly (monoubiquitination) or in chains (polyubiquitination), is used to regulate diverse processes from protein degradation to gene expression to the cell cycle.[3]

References

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[1] Walsh C. Posttranslational Modification of Proteins: Expanding Nature’s Inventory. Englewood, CO: Roberts and Co. Publishers; 2006.

[2] Glozak MA, Sengupta N, Zhang X, Seto E. Acetylation and deacetylation of non-histone proteins. Gene. 2005;363:15–23. https://doi.org/10.1016/j. gene.2005.09.010.

[3] Oh E, Akopian D, Rape M. Principles of ubiquitin-dependent signaling. Annu Rev Cell Dev Biol. 2018;34:137–162. https://doi.org/10.1146/ annurev-cellbio-100617-062802.

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مواضيع ذات صلة

The Protein Kinase Domain

The Double att Phage, att2

Transducing Phage Carrying Genes Other than gal and bio

Incorrect Excision and gal and bio Transducing Phage

Isolation of Excision-Deficient Mutants

Amplification Techniques in Molecular Biology

Characteristics of Nucleic Acids

solation of Integration-Defective Mutants

Gene Order Permutation and the Campbell Model

Simultaneous Deletion of Chromosomal and Lambda DNA

Mapping Integrated Lambda

Expression Patterns of Developmental Genes