Post-Translational Modifications (Part 1)

Phosphorylation is the most well-studied PTM, with roles in many essential cellular processes. These include normal cell growth and division, apoptosis, and countless signal transduction pathways, of which the mitogen-activated protein kinase (MAPK) cascade, the mammalian target of rapamycin (mTOR) pathway, and the Janus kinase (JAK) signal transducer and activator of transcription (JAK-STAT) pathway are some of the best-known examples.

Phosphorylation is catalyzed by kinase enzymes, which transfer a phosphate group from ATP to amino acids with a hydroxyl group in their side chains (predominantly tyrosine, threonine, and serine residues). Dephosphorylation is catalyzed by phosphatases. The dbPTM databank shows approximately 1,900 phosphorylation sites to be associated with disease traits³. This equates to almost 80 kinase inhibitors having so far been approved for clinical use, mainly for the treatment of cancers⁵.

Ubiquitin is a 76 amino acid protein that can be attached singly to target molecules (monoubiquitination) or can form polymeric chains through ubiquitination of its seven internal lysine residues. While monoubiquitination is generally regarded as a signal for non-proteolytic events such as endocytosis, histone regulation, and DNA repair, polyubiquitination (especially Lys48- and Lys29-linked polyubiquitination) is associated with degradation of target proteins by the 26S proteasome⁶.

The ubiquitination process involves a 3-step enzymatic cascade, during which an enzyme complex containing ubiquitin-activating (E1), ubiquitin-conjugating (E2), and ubiquitin ligase (E3) enzymes attaches ubiquitin via its C-terminal glycine to the ε-amino group of a lysine residue on the substrate. The reverse reaction, ubiquitin removal, is performed by deubiquitinases. Dysfunction in the ubiquitin pathway has been linked to conditions including congenital developmental defects, cancer, and immune disorders.

Acetylation is important for chromatin stability, protein–protein interaction, and cell cycle control, as well as for cell metabolism, nuclear transport, and actin nucleation⁴. However, a main reason for studying acetylation is to uncover its role in the histone code, a hypothesis which states that DNA transcription is largely regulated by post-translational modifications to histone proteins⁷.

Acetylation is catalyzed by lysine acetyltransferases (KATs), which use acetyl CoA as a co-factor for adding an acetyl group to the ε-amino group of lysine side chains. It is reversed by lysine deacetylases (KDACs). Dysregulated acetylation has been associated with cardiovascular disease, neurodegenerative disorders, and cancer. In addition, it was recently discovered that SARS-CoV-2 nucleocapsid proteins are strongly acetylated by human acetyltransferase enzymes, which may influence their functions⁸.