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N-terminus

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Title: N-terminus  
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Subject: Ubiquitin, DNA clamp, C-terminus, P14arf, Importin
Collection: Posttranslational Modification, Protein Structure, Proteins
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N-terminus

A tetrapeptide (example: Val-Gly-Ser-Ala) with green highlighted N-terminal α-amino acid (example: L-valine) and blue marked C-terminal α-amino acid (example: L-alanine). This tetrapeptide could be encoded by the mRNA sequence 5'-GUUGGUAGUGCU-3'.

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). By convention, peptide sequences are written N-terminus to C-terminus, left to right in LTR languages. This correlates the translation direction to the text direction (because when a protein is translated from messenger RNA, it is created from N-terminus to C-terminus - amino acids are added to the carbonyl end).

Contents

  • Chemistry 1
  • Function 2
    • N-Terminal targeting signals 2.1
      • Signal peptide 2.1.1
      • Mitochondrial targeting peptide 2.1.2
      • Chloroplast targeting peptide 2.1.3
    • N-Terminal modifications 2.2
      • N-Myristoylation 2.2.1
      • N-Acylation 2.2.2
      • N-Terminal acetylation 2.2.3
  • See also 3
  • References 4

Chemistry

Each amino acid has an amine group and a carboxylic group. A chain of amino acids are linked by peptide bonds 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. Thus, a polypeptide chain has two ends - an amine group, the N-terminus, and an unbound carboxyl group, the C-terminus.[1]

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 tRNA) 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 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.

Function

N-Terminal targeting signals

The N-terminus is the first part of the protein that exits the 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.

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.

N-Terminal acetylation

N-terminal acetylation

is a form of protein modification that can occur in both prokaryotes and eukaryotes. It has been suggested that N-terminal acetylation can prevent a protein from following a secretory pathway.[2]

See also

  • TopFIND, a scientific database covering proteases, their cleavage site specificity, substrates, inhibitors and protein termini originating from their activity

References

  1. ^ Voet, Donald; Voet, Judith G.; Pratt, Charlotte W. (2013). Fundamentals of Biochemistry: Life at the Molecular Level (4th ed.). Hoboken, NJ: Wiley.  
  2. ^ Arnesen, Thomas (May 31, 2011). "Towards a Functional Understanding of Protein N-Terminal Acetylation". PLoS Biology.  
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