The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amine group is bonded to the carboxylic acid group of another amino acid, making it a chain. That leaves a free carboxylic group at one end of the peptide, called the C-terminus, and a free amine group on the other end called the N-terminus. By convention, peptide sequences are written N-terminus to C-terminus, left to right (in LTR writing systems).
Chemistry
Each amino acid has an
amine group and a
carboxylic group. Amino acids link to one another by
which form through a dehydration reaction that joins the carboxyl group of one amino acid to the
amine group of the next in a head-to-tail manner to form a
Peptide chain. The chain has two ends – an amine group, the N-terminus, and an unbound carboxyl group, the
C-terminus.
When a protein is translated from messenger RNA, it is created from N-terminus to C-terminus. The amino end of an amino acid (on a charged transfer RNA) during the elongation stage of translation, attaches to the carboxyl end of the growing chain. Since the start codon of the genetic code codes for the amino acid methionine, most protein sequences start with a methionine (or, in bacteria, mitochondria and , the modified version N-formylmethionine, fMet). However, some proteins are modified posttranslationally, for example, by cleavage from a protein precursor, and therefore may have different amino acids at their N-terminus.
Function
N-terminal targeting signals
The N-terminus is the first part of the protein that exits the
ribosome during protein biosynthesis. It often contains
signal peptide sequences, "intracellular
" that direct delivery of the protein to the proper
organelle. The signal peptide is typically removed at the destination by a signal
peptidase. The N-terminal amino acid of a protein is an important determinant of its half-life (likelihood of being degraded). This is called the
N-end rule.
Signal peptide
The N-terminal signal peptide is recognized by the signal recognition particle (SRP) and results in the targeting of the protein to the secretory pathway. In
eukaryote, these proteins are synthesized at the rough endoplasmic reticulum. In
prokaryote, the proteins are exported across the
cell membrane. In
, signal peptides target proteins to the
.
Mitochondrial targeting peptide
The N-terminal mitochondrial
target peptide (mtTP) allows the protein to be imported into the
mitochondrion.
Chloroplast targeting peptide
The N-terminal chloroplast targeting peptide (cpTP) allows for the protein to be imported into the
chloroplast.
N-terminal modifications
Protein N-termini can be modified co - or post-translationally. Modifications include the removal of initiator methionine (iMet) by
, attachment of small chemical groups such as
Acetyl group,
propionyl and
Methyl group, and the addition of membrane anchors, such as
Palmitoyl group and
N-terminal acetylation
N-terminal acetylation is a form of protein modification that can occur in both
and
. It has been suggested that N-terminal acetylation can prevent a protein from following a secretory pathway.
N-Myristoylation
The N-terminus can be modified by the addition of a myristoyl anchor. Proteins that are modified this way contain a consensus motif at their N-terminus as a modification signal.
N-Acylation
The N-terminus can also be modified by the addition of a
fatty acid anchor to form N-acetylated proteins. The most common form of such modification is the addition of a palmitoyl group.
See also
-
C-terminus
-
TopFIND, a scientific database covering , their cleavage site specificity, substrates, inhibitors and protein termini originating from their activity
