nterminus

An N-terminus, or amino terminus, is the start of a protein or polypeptide, representing the first amino acid in the chain. It has a free amino group and is the site where synthesis begins on the ribosome. In most eukaryotic proteins, the initial residue is methionine, which can be removed by methionine aminopeptidase depending on the identity of the second residue; in bacteria, initiation typically uses formylmethionine, which may also be cleaved in mature proteins.

The N-terminus is a common site for post-translational modification and processing. N-terminal acetylation, carried out by N-terminal acetyltransferases such as NatA, NatB, and NatC, is among the most frequent protein modifications and can influence stability, localization, and interactions. N-myristoylation adds a myristoyl group to an exposed N-terminal glycine after removal of the initiator methionine, often promoting membrane association. Some proteins undergo removal of signal or transit peptides at the N-terminus to yield the mature protein, exposing a new N-terminus that can affect localization or function.

The N-terminus can influence protein fate through the N-end rule pathway, where certain N-terminal residues act as degradation signals, thereby affecting half-life and turnover. The precise composition of the N-terminus can also impact subcellular localization and protein–protein interactions.

Analyses of N-termini are central to proteomics and protein sequencing. Historically, Edman degradation was used to determine the N-terminal sequence; modern approaches rely on mass spectrometry and specialized N-terminomics techniques to identify and quantify N-terminal residues and modifications.