The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy 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 the protein is translated from messenger RNA, it is created from N-terminus to C-terminus. 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. Amino acids link to one another to form a chain by a dehydration reaction which joins 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 unbound amine group, the
N-terminal end. Proteins are naturally 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 (
lysine-
Aspartic acid-
Glutamic acid-
Leucine) or -HDEL (
Histidine-Asp-Glu-Leu) at the C-terminus. This keeps the protein in the endoplasmic reticulum and prevents it from entering the secretory pathway.
Peroxisomal targeting signal
The sequence -SKL (Ser-Lys-Leu) or similar near C-terminus serves as peroxisomal targeting signal 1, directing the protein into
peroxisome.
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
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
are often modified this way.
GPI anchors
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.
Methylation
C-terminal
leucine is methylated at carboxyl group by enzyme leucine carboxyl methyltransferase 1 in vertebrates, forming
methyl ester.
C-terminal domain
The C-terminal domain of some proteins has specialized functions. In humans, the CTD of RNA polymerase II typically consists of up to 52 repeats of the sequence
Tyrosine-Ser-
Proline-
Threonine-Ser-Pro-Ser.
This allows other proteins to bind to the C-terminal domain of RNA polymerase in order to activate polymerase activity. These domains are then involved in the initiation of DNA transcription, the
capping enzyme of the
Messenger RNA, and attachment to the
spliceosome for
RNA splicing.
See also
-
N-terminus
-
TopFIND, a scientific database covering , their cleavage site specificity, substrates, inhibitors and protein termini originating from their activity
