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.
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Chemistry
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 sequences that act as targeting signals, basically intracellular zip codes, that allow for the protein to be delivered to its designated location within the cell. The targeting signal is usually cleaved off after successful targeting by a processing peptidase
- 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.
- 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-acylated proteins. The most common form of such modification is the addition of a palmitoyl group.
See also
References
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