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The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) refers to the start of a protein or polypeptide terminated by an amino acid with a free amine group (-NH2). The convention for writing peptide sequences is to put the N-terminus on the left and write the sequence from N- to C-terminus. When the protein is translated from messenger RNA, it is created from N-terminus to C-terminus.
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.
When the 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 tRNA) during the elongation stage of translation, attaches to the carboxyl end of the growing or nascent 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 chloroplasts, 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.
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 postal codes" 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 eukaryotic cells, these proteins are synthesized at the rough endoplasmic reticulum. In prokaryotic cells, the proteins are exported across the cell membrane. In chloroplasts, signal peptides target proteins to the thylakoids.
Mitochondrial targeting peptide 
The N-terminal mitochondrial targeting peptide (mtTP) allows for 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 
Some proteins are modified posttranslationally by the addition of membrane anchors that allow the protein to associate with membrane without having a transmembrane domain. The N-terminus (as well as the C-terminus) of a protein can be modified this way.
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.
See also 
- TopFIND, a scientific database covering proteases, their cleavage site specificity, substrates, inhibitors and protein termini originating from their activity