carboxyterminal

The carboxyterminal, commonly called the C-terminus, is the end of a polypeptide or protein that bears a free carboxyl group. It lies opposite the amino terminus (N-terminus) and defines the sequence from N to C during translation. The C-terminus can be unmodified or subjected to various post-translational modifications that influence protein behavior.

At the chemical level, the C-terminus ends in a free carboxyl group and may be modified after translation. C-terminal amidation is common in neuropeptides; glycine residues often serve as amide donors. C-terminal processing by proteases can generate mature forms by removing extensions. The extreme tail can contain targeting signals such as KDEL for endoplasmic reticulum retention or SKL (PTS1) for peroxisomal targeting, or interaction motifs like PDZ-binding sequences that mediate protein interactions.

Functional significance includes roles in subcellular localization, protein stability, and interactions with other molecules. The concept of a C-end degron describes signals at the C-terminus that mark proteins for ubiquitin-mediated degradation by specific E3 ligases. C-terminal modifications and motifs can also modulate activity and half-life, influencing how a protein functions in a cellular context.

In biotechnology and research, the C-terminus is a common place to add affinity or detection tags (C-terminal tagging) for purification and analysis. Tags at the C-terminus can affect folding, localization, or function, so researchers often design variants with or without C-terminal tags and may include proteolytic sites to remove the tag after purification.