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The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal end, or COOH-terminus) of a protein or polypeptide is the end of the amino acid chain terminated by a free carboxyl group (-COOH). The convention for writing peptide sequences is to put the C-terminal end on the right and write the sequence from N- to C-terminus.

Chemistry

Each amino acid has a carboxyl group and an amine group, and amino acids link to one another to form a chain by a dehydration reaction by joining the amine group of one amino acid to the carboxyl group of the next. Thus polypeptide chains have an end with an unbound carboxyl group, the C-terminus, and an end with an amine group, the N-terminus. Proteins are synthesized starting from the N-terminus and ending at the C-terminus.

Function

C-terminal retention signals

While the N-terminus of a protein often contains targeting signals, the C-terminus can contain retention signals for protein sorting. The most common ER retention signal is the amino acid sequence -KDEL (or -HDEL) at the C-terminus, which keeps the protein in the endoplasmic reticulum and prevents it from entering the secretory pathway.

C-terminal modifications

The C-terminus of proteins can be modified posttranslationally, most commonly by the addition of a lipid anchor to the C-terminus that allows the protein to be inserted into a membrane without having a transmembrane domain.

• Prenylation

Main article: Prenylation

One form of C-terminal modification is prenylation. During prenylation, a farnesyl- or geranylgeranyl-isoprenoid membrane anchor is added to a cysteine residue near the C-terminus. Small, membrane-bound G proteins are often modified this way.

• GPI anchors

Main article: Glycosylphosphatidylinositol

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 a C-terminal propeptide. The most prominent example for this type of modification is the prion protein.

C-terminal domain or Carboxyl Tail Domain

The C-terminal domain (CTD) of some proteins has specialized functions.

RNA POL II in action.

• CTD of RNA polymerase

The carboxy-terminal domain of RNA polymerase II typically consists of up to 52 repeats of the sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser ¹. Other proteins often bind the C-terminal domain of RNA polymerase in order to activate polymerase activity. It is the protein domain which is involved in the initiation of DNA transcription, the capping of the RNA transcript, and attachment to the spliceosome for RNA splicing².

See also

• N-terminus

References

1. ^ Nature
2. ^ Functional study of the CTD in Drosophila

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