N-terminus

"N-term" redirects here. For the divergence test, seeTerm test.

Atetrapeptide(example:Val-Gly-Ser-Ala) withgreenhighlighted N-terminal α-amino acid (example: L-valine) andbluemarked C-terminal α-amino acid (example: L-alanine). This tetrapeptide could be encoded by the mRNA sequence 5'-GUUGGUAGUGCU-3'.

TheN-terminus(also known as theamino-terminus,NH₂-terminus,N-terminal endoramine-terminus) is the start of aproteinorpolypeptide,referring to the freeaminegroup (-NH₂) located at the end of a polypeptide. Within a peptide, the amine group is bonded to thecarboxylicgroup of another amino acid, making it a chain. That leaves a free carboxylic group at one end of the peptide, called theC-terminus,and a free amine group on the other end called the N-terminus. By convention, peptide sequences are written N-terminus toC-terminus,left to right (inLTR writing systems).This correlates thetranslationdirection to the text direction, because when a protein is translated frommessenger RNA,it is created from the N-terminus to the C-terminus, as amino acids are added to the carboxyl end of the protein.

Chemistry

Each amino acid has anaminegroup and acarboxylic group.Amino acids link to one another bypeptide bondswhich form through adehydration reactionthat joins the carboxyl group of one amino acid to theaminegroup of the next in a head-to-tail manner to form apolypeptidechain. The chain has two ends – an amine group, the N-terminus, and an unbound carboxyl group, theC-terminus.

When a protein istranslatedfrommessenger RNA,it is created from N-terminus to C-terminus. The amino end of an amino acid (on a chargedtRNA) during the elongation stage of translation, attaches to the carboxyl end of the growing chain. Since thestart codonof thegenetic codecodes for the amino acidmethionine,most protein sequences start with amethionine(or, in bacteria,mitochondriaandchloroplasts,the modified versionN-formylmethionine,fMet). However, some proteins are modifiedposttranslationally,for example, by cleavage from aprotein 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 theribosomeduringprotein biosynthesis.It often containssignal peptidesequences, "intracellularpostal codes"that direct delivery of the protein to the properorganelle.The signal peptide is typically removed at the destination by a signalpeptidase.The N-terminal amino acid of a protein is an important determinant of its half-life (likelihood of being degraded). This is called theN-end rule.

Signal peptide

Main article:Signal peptide

The N-terminal signal peptide is recognized by thesignal recognition particle(SRP) and results in the targeting of the protein to thesecretory pathway.Ineukaryotic cells,these proteins are synthesized at the roughendoplasmic reticulum.Inprokaryotic cells,the proteins are exported across thecell membrane.Inchloroplasts,signal peptides target proteins to thethylakoids.

Mitochondrial targeting peptide

The N-terminal mitochondrialtargeting peptide(mtTP) allows the protein to be imported into themitochondrion.

Chloroplast targeting peptide

The N-terminal chloroplast targeting peptide (cpTP) allows for the protein to be imported into thechloroplast.

N-terminal modifications

Protein N-termini can be modified co - or post-translationally. Modifications include the removal of initiator methionine (iMet) byaminopeptidases,attachment of small chemical groups such asacetyl,propionylandmethyl,and the addition of membrane anchors, such aspalmitoylandmyristoyl groups

N-terminal acetylation

Main article:N-terminal acetylation

N-terminal acetylationis a form of protein modification that can occur in bothprokaryotesandeukaryotes.It has been suggested that N-terminal acetylation can prevent a protein from following asecretory pathway.

N-Myristoylation

Main article: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

Main article:Palmitoylation

The N-terminus can also be modified by the addition of afatty acidanchor 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 coveringproteases,their cleavage site specificity, substrates, inhibitors and protein termini originating from their activity