target peptide

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Target peptide

From Wikipedia, the free encyclopedia

A target peptide is a short (3-60 amino acids long) peptide chain that directs the transport of a

protein to a specific region in the cell, including the nucleus, mitochondria, endoplasmic reticulum(ER), chloroplast, apoplast, peroxisome and plasma membrane. Some target peptides are cleaved

from the protein by signal peptidases after the proteins are transported.

Contents [hide]

1 Types by protein destination

1.1 Secretion

1.2 ER-Retention Signal

1.3 Nucleus

1.4 Nucleolus

1.5 Mitochondria

1.6 Peroxisome

2 Examples of target peptides

3 See also

4 References

5 External links

[edit]Types by protein destination

[edit]Secretion

Main article: signal peptide

Almost all proteins that are destined to the secretory pathway have a sequence consisting of 5-20

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 ineukaryotes, where the signal peptide directs the newly synthesized protein to the Sec61 channel,


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which shares structural and sequence homology with SecYEG, but is present in the endoplasmic

reticulum.[1] 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.

[edit]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, KDEL (lys-asp-

glu-leu), on their C-terminus, they are retained in the ER or are routed back to the ER (in instances

where they escape) via interaction with the KDEL receptor in the Golgi apparatus.

[edit]Nucleus

Main article: nuclear localization 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.

[edit]Nucleolus

The nucleolus within the nucleus can be targeted with a sequence called a nucleolar localization

signal (abbreviated NoLS or NOS).

[edit]Mitochondria

The mitochondrial targeting signal is a 10-60 amino acid long peptide that directs a newly

synthesized proteins to the mitochondria. It is found at the N-terminus and 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.

Like signal peptides, mitochondrial targeting signals are cleaved once targeting is complete.

[edit]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.


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Signal peptide

From Wikipedia, the free encyclopedia

Identifiers

OPM superfamily 292

OPM protein 1skh

A signal peptide (sometimes referred to as signal sequence, leader sequence or leader peptide) is a

short (5-30 amino acids long) peptide present at the N-terminus of the majority of newly synthesized

proteins that are destined towards the secretory pathway.[1] These proteins include those that

reside either inside certain organelles (the endoplasmic reticulum, golgi or endosomes), secreted

from the cell, or inserted into most cellular membranes. Although most type I membrane-bound

proteins have signal peptides, the majority of type II and multi-spanning membrane-bound proteins

are targeted to the secretory pathway by their first transmembrane domain, which biochemically

resembles a signal sequence except that it is not cleaved.

Contents [hide]

1 Translocation

2 Signal Peptide Structure

3 Co-translational versus Post-translational Translocation

4 Signal Peptides are determining the Secretion Efficiency

5 Nucleotide Level Features

6 See also

7 References

8 External links

[edit]Translocation

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 homology with SecYEG, but is present in the endoplasmic

reticulum.[2] 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.

[edit]Signal Peptide Structure


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The core of the signal peptide contains a long stretch of hydrophobic amino acids that has a

tendency to form a single alpha-helix. In addition, many signal peptides begin with a short positively

charged stretch of amino acids, which may help to enforce proper topology of the polypeptide

during translocation by what is known as the positive-inside rule.[3] At the end of the signal peptide

there is typically a stretch of amino acids that is recognized and cleaved by signal peptidase.

However this cleavage site is absent from transmembrane-domains that serve as signal peptides,

which are sometimes referred to as signal anchor sequences. Signal peptidase may cleave either

during or after completion of translocation to generate a free signal peptide and a mature protein.

The free signal peptides are then digested by specific proteases.

[edit]Co-translational versus Post-translational Translocation

In both prokaryotes and eukaryotes signal sequences may act co-translationally or post-

translationally.

The co-translational pathway is initiated when the signal peptide emerges from the ribosome and is

recognized by the signal-recognition particle (SRP).[4] SRP then halts further translation and directs

the signal sequence-ribosome-mRNA complex to the SRP receptor, which is present on the surface

of either the plasma membrane (in prokaryotes) or the ER (in eukaryotes).[5] Once membrane-

targeting is completed, the signal sequence is inserted into the translocon. Ribosomes are then

physically docked onto the cytoplasmic face of the translocon and protein synthesis resumes.[6]

The post-translational pathway is initiated after protein synthesis is completed. In prokaryotes, the

signal sequence of post-translational substrates is recognized by the SecB chaperone protein that

transfers the protein to the SecA ATPase, which in turns pumps the protein through the translocon.

Although post-translational translocation is known to occur in eukaryotes, it is poorly understood. It

is however known that in yeast post-translational translocation requires the translocon and two

additional membrane-bound proteins, Sec62 and Sec63.[7]

[edit]Signal Peptides are determining the Secretion Efficiency

Signal peptides are extremely heterogeneous and many prokaryotic and eukaryotic signal peptides

are functionally interchangeable even between different species however the efficiency of protein

secretion is stongly determined by the signal peptide.[8] [9]

[edit]Nucleotide Level Features

In vertebrates, the region of the mRNA that codes for the signal peptide (i.e. the signal sequence

coding region, or SSCR) can function as an RNA element with specific activities. SSCRs promotenuclear mRNA export and the proper localization to the surface of the endoplasmic reticulum. In


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addition SSCRs have specific sequence features: they have low adenine-content, are enriched in

certain motifs, and tend to be present in the first exon at a frequency that is higher than

expected.[10][11]

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