Myristyl, also known as N-myristoyl, is a peptide modification that consists of a 14-carbon myristic acid chain covalently attached to the amino acid residue, most commonly the N-terminal glycine residue, through an amide bond. The myristic acid chain acts as a lipophilic (fat-loving) anchor, which allows the peptide to interact with and insert into the hydrophobic (water-fearing) regions of biomembranes.

Myristylation is a post-translational modification that occurs in a variety of eukaryotic and prokaryotic organisms. In eukaryotes, the myristoylation process is mediated by a family of enzymes called N-myristoyltransferases (NMTs), which transfer the myristic acid from a cofactor, coenzyme A, to the N-terminal glycine residue of the target peptide. The myristoylation process is essential for the proper localization, stability, and activity of many eukaryotic proteins, including signal transduction molecules, ion channels, and enzymes.

One of the best-studied examples of myristylation is the protein G protein-coupled receptors (GPCRs). GPCRs are a large family of cell surface receptors that sense a wide variety of extracellular signals and transmit the signal to the intracellular signaling pathways. GPCRs are typically composed of seven transmembrane (TM) helices, and the N-terminal helix is myristylated in most GPCRs. The myristylation of the N-terminal helix is important for the proper folding and stability of GPCRs, as well as for their ability to interact with G proteins.

Another example of myristylation is the protein Ras, which is a small GTPase that plays a critical role in intracellular signal transduction pathways. Ras is myristylated at its N-terminal glycine residue, which is essential for its localization to the plasma membrane and its ability to interact with downstream effectors.

Myristylation also plays an important role in the regulation of enzymes. For example, the enzyme phosphatidylinositol 3-kinase (PI3K) is myristylated at its N-terminal glycine residue, which is essential for its localization to the plasma membrane and its ability to phosphorylate phosphatidylinositol, a phospholipid that is a key player in intracellular signaling pathways.

In addition to its role in intracellular signaling pathways, myristylation also plays a role in the regulation of the immune system. For example, the protein NKT is a T cell receptor that recognizes and binds to a specific glycolipid antigen presented by dendritic cells. NKT is myristylated at its N-terminal glycine residue, which is essential for its ability to bind to the glycolipid antigen.

Myristylation also plays a role in the regulation of transcription. For example, the protein p53 is a transcription factor that plays a critical role in the regulation of cell growth and proliferation. p53 is myristylated at its N-terminal glycine residue, which is essential for its stability and ability to bind to DNA.

In conclusion, myristylation is a post-translational modification that plays a critical role in the proper localization, stability, and activity of many eukaryotic proteins, including signal transduction molecules, ion channels, enzymes and transcription factors.