Target peptide

Types by protein destination

Secretion

Main article: signal peptide

Almost all proteins that are destined to the secretory pathway have a sequence consisting of 5-30 hydrophobic amino acids on the N-terminus, which is commonly referred to as the signal peptide, signal sequence or leader peptide. Signal peptides form alpha-helical structures. Proteins that contain such signals are destined for either extra-cellular secretion, the plasma membrane, the lumen or membrane of either the (ER), Golgi or endosomes. Certain membrane-bound proteins are targeted to the secretory pathway by their first transmembrane domain, which resembles a typical signal peptide.

In prokaryotes, signal peptides direct the newly synthesized protein to the SecYEG protein-conducting channel, which is present in the plasma membrane. A homologous system exists in eukaryotes, where the signal peptide directs the newly synthesized protein to the Sec61 channel, which shares structural and sequence similarity with SecYEG, but is present in the endoplasmic reticulum. Both the SecYEG and Sec61 channels are commonly referred to as the translocon, and transit through this channel is known as translocation. While secreted proteins are threaded through the channel, transmembrane domains may diffuse across a lateral gate in the translocon to partition into the surrounding membrane.

ER-Retention Signal

In eukaryotes, most of the newly synthesized secretory proteins are transported from the ER to the Golgi apparatus. If these proteins have a particular 4-amino-acid retention sequence for the ER's lumen, KDEL, on their C-terminus, they are retained in the ER's lumen or are routed back to the ER's lumen (in instances where they escape) via interaction with the KDEL receptor in the Golgi apparatus. If the signal is KKXX, the retention mechanism to the ER will be similar but the protein will be transmembranal.

Nucleus

Main article: nuclear localization signal}}{{main, nuclear export signal

A nuclear localization signal (NLS) is a target peptide that directs proteins to the nucleus and is often a unit consisting of five basic, positively charged amino acids. The NLS normally is located anywhere on the peptide chain.

A nuclear export signal (NES) is a target peptide that directs proteins from the nucleus back to the cytosol. It often consists of several hydrophobic amino acids (often leucine) interspaced by 2-3 other amino acids.

Many proteins are known to constantly shuttle between the cytosol and nucleus and these contain both NESs and NLSs.

Nucleolus

The nucleolus within the nucleus can be targeted with a sequence called a nucleolar localization signal (abbreviated NoLS or NOS).

Mitochondria and plastid

The mitochondrial targeting signal also known as presequence is a 10-70 amino acid long peptide that directs a newly synthesized protein to the mitochondria. It is found at the N-terminus end consists of an alternating pattern of hydrophobic and positively charged amino acids to form what is called an amphipathic helix. Mitochondrial targeting signals can contain additional signals that subsequently target the protein to different regions of the mitochondria, such as the mitochondrial matrix or inner membrane. In plants, an N-terminal signal (or transit peptide) targets to the plastid in a similar manner. Like most signal peptides, mitochondrial targeting signals and plastid specific transit peptides are cleaved once targeting is complete. Some plant proteins have an N-terminal transport signal that targets both organelles often referred to as dual-targeted transit peptide. Approximately 5% of total organelle proteins are predicted to be dual-targeted however the specific number could be higher considering the variable degree of accumulation of passenger proteins in both organelles. The targeting specificity of these transit peptides depends on many factors including net charge and affinity between transit peptides and organelle transport machinery.

Peroxisome

There are two types of target peptides directing to peroxisome, which are called peroxisomal targeting signals (PTS). One is PTS1, which is made of three amino acids on the C-terminus. The other is PTS2, which is made of a 9-amino-acid sequence often present on the N-terminus of the protein.

Examples of target peptides

The following content uses protein primary structure single-letter location. A "[n]" prefix indicates the N-terminus and a "[c]" suffix indicates the C-terminus; sequences lacking either are found in the middle of the protein.

References

References

1. Rapoport T.. (Nov 2007). "Protein translocation across the eukaryotic endoplasmic reticulum and bacterial plasma membranes.". Nature.
2. (March 2003). "KDEL and KKXX Retrieval Signals Appended to the Same Reporter Protein Determine Different Trafficking between Endoplasmic Reticulum, Intermediate Compartment, and Golgi Complex". Molecular Biology of the Cell.
3. (February 2013). "A reevaluation of dual-targeting of proteins to mitochondria and chloroplasts". Biochimica et Biophysica Acta (BBA) - Molecular Cell Research.
4. (December 2018). "Rather rule than exception? How to evaluate the relevance of dual protein targeting to mitochondria and chloroplasts". Photosynthesis Research.
5. (2019-06-01). "Targeting specificity of nuclear-encoded organelle proteins with a self-assembling split-fluorescent protein toolkit". Journal of Cell Science.
6. (January 2014). "The exception proves the rule? Dual targeting of nuclear-encoded proteins into endosymbiotic organelles". New Phytologist.
7. (January 2014). "Import Determinants of Organelle-Specific and Dual Targeting Peptides of Mitochondria and Chloroplasts in Arabidopsis thaliana". Molecular Plant.
8. "Pepscan Motifs".
9. "Subcellular localization signal".
10. (2006). "The bacterial twin-arginine translocation pathway.". Annual Review of Microbiology.