The C terminus (also known as carboxyl terminus , carboxy terminus , C-terminal tail , C-terminal end , or COOH terminus ) is the end of an amino acid chain ( protein or polypeptide ), terminated by a free carboxyl group ( -COOH). When a protein is translated from messenger RNA, it is made from the N-terminus to the Cterminus. The convention for writing a peptide sequence is to place the C-terminal end to the right and write the sequence from N- to Cterminus.
Each amino acid contains a carboxyl group and an amine group. Amino acids link to each other by a dehydration reaction to form a chain that links the amine group of one amino acid to the carboxyl group of the next. Thus polypeptide chains end with an unbound carboxyl group, the Cterminus, and an unbound amine group, the N-terminus . Proteins naturally start at the N-terminus and end at the Cterminus.
C-terminal retention signal
Whereas the N-terminus of proteins often contains targeting signals, the C-terminus may contain retention signals for protein sorting. The most common ER retention signal is the amino acid sequence -KDEL (Lis-Asp-Glu-Leu) or -HDEL (His-ASP-Glu-Leu) at the C-terminus. This keeps the protein in the endoplasmic reticulum and prevents it from entering the secretory pathway.
The C-terminus of a protein can be modified posttranslationally , usually by adding a lipid anchor at the Cterminus, which allows the protein to be inserted into a membrane without a transmembrane domain .
One form of C-terminal modification is prenylation . During prenylation , a farnesyl- or geranylgeranyl- isoprenoid is added to a cysteine residue near the membrane anchor C terminus. Small, membrane-bound G proteins are often modified in this way.
Another form of C-terminal modification is the addition of a phosphoglycan, glycosylphosphatidylinositol (GPI) , as a membrane anchor . The GPI anchor is attached to the C-terminus after proteolytic cleavage of the C-terminal propeptide. The most prominent example for this type of modification is the prion protein.
The C-terminal domain of some proteins has specialized functions. In humans, the CTD of RNA polymerase II typically consists of 52 repeats of the Tyr–Ser–Pro–Thr–Ser–Pro–Ser sequence.  This allows other proteins to bind to the C-terminal domain of RNA polymerase to activate polymerase activity. These domains are then involved in the initiation of DNA transcription, the capping of the RNA transcript , and the attachment of the spliceosome to RNA splicing .