Protein targeting
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2008-06-14T20:37:00Z
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:''This article deals with protein targeting in [[eukaryote]]s except where noted.''
'''Protein targeting''' or '''protein sorting''' is the mechanism by which a [[cell (biology)|cell]] transports [[protein]]s to the appropriate positions in the cell or outside of it. Sorting targets can be the inner space of an [[organelle]], any of several interior [[biological membrane|membrane]]s, the cell's [[cell membrane|outer membrane]], or its exterior via [[secretion]]. This delivery process is carried out based on information contained in the protein itself. Correct sorting is crucial for the cell; errors can lead to diseases.
== Targeting signals ==<!-- This section is linked from [[Carboxypeptidase E]] -->
{{main|Signal peptide}}
''Targeting signals'' are the pieces of information that enable the cellular transport machinery to correctly position a protein inside or outside the cell. This information is contained in the [[primary structure|polypeptide chain]] or in the folded protein. The continuous stretch of [[amino acid residue]]s in the chain that enables targeting are called [[signal peptide]]s or [[targeting peptide]]s. There are two types of targeting peptides, the presequences and the internal targeting peptides. The presequences of the targeting peptide are often found at the N-terminal extension and is composed of between 6-136 basic and hydrophobic amino acids.In case of peroxisomes the targeting sequence is on the C-terminal extension mostly. Other signals are composed by parts which are separate in the [[primary sequence]]. To function these components have to come together on the protein surface by [[protein folding|folding]]. They are called [[signal patches]]. In addition, [[posttranslational modification|protein modifications]] like glycosylations can induce targeting.
==Protein translocation==
In [[1970]], [[Günter Blobel]] conducted experiments on the translocation of proteins across membranes. He was awarded the [[1999]] [[Nobel Prize/Physiology or medicine|Nobel prize]] for his findings. He discovered that many proteins have a [[signal peptide|signal sequence]], that is, a short [[amino acid]] sequence at one end that functions like a [[postal code]] for the target organelle. The [[translation (biology)|translation]] of [[mRNA]] into protein by a [[ribosome]] takes place within the [[cytosol]]. If the synthesized proteins "belong" in a different organelle, they can be transported there in either of two ways, depending on the protein.
===Cotranslational translocation===
The N-terminal signal sequence of the protein is recognized by a [[signal recognition particle]] (SRP) ''while the protein is still being synthesized on the ribosome''. The synthesis pauses while the ribosome-protein complex is transferred to an [[SRP receptor]] on the [[endoplasmic reticulum]] (ER), a membrane-enclosed organelle. There, the nascent protein is inserted into the [[Sec61|Sec61 translocation complex]] (also known as the translocon) that passes through the ER membrane. The signal sequence is immediately cleaved from the polypeptide once it has been translocated into the ER by [[signal peptidase]] in secretory proteins. This signal sequence processing differs for some ER [[transmembrane proteins]]. Within the ER, the protein is first covered by a [[chaperone (protein)|chaperone protein]] to protect it from the high concentration of other proteins in the ER, giving it time to [[protein folding|fold]] correctly. Once folded, the protein is modified as needed (for example, by [[glycosylation]]), then transported to the [[Golgi apparatus]] for further processing and goes to its target organelles or is retained in the ER by various [[ER retention]] mechanisms.
===Posttranslational translocation===
Even though most proteins are cotranslationally translocated, some are translated in the [[cytosol]] and later transported to their destination. This occurs for proteins that go to a [[mitochondrion]], a [[chloroplast]], or a [[peroxisome]] (proteins that go to the latter have their signal sequence at the C terminus). Also, proteins targeted for the [[cell nucleus|nucleus]] are translocated post-translation. They pass through the [[nuclear envelope]] via [[nuclear pore]]s.
===Transmembrane proteins===
The amino acid chain of [[transmembrane protein]]s, which often are [[transmembrane receptor]]s, passes through a membrane one or several times. They are inserted into the membrane by translocation, until the process is interrupted by a stop-transfer sequence, also called a membrane anchor sequence.
These complex membrane proteins are at the moment mostly understood using the same model of targeting that has been developed for secretory proteins. However, many complex multi-transmembrane proteins contain structural aspects that do not fit the model. Seven transmembrane G-protein coupled receptors (which represent about 5% of the genome of humans) mostly do not have an amino-terminal signal sequence. In contrast to secretory proteins, the first transmembrane domain acts as the first signal sequence, which targets them to the ER membrane. This also results in the translocation of the amino terminus of the protein into the ER membrane lumen. This would seem to break the rule of "co-translational" translocation which has always held for mammalian proteins targeted to the ER. This has been demonstrated with [[opsin]] with in vitro experiments.<ref>{{cite journal | author =Kanner EM, Friedlander M, Simon SM. | title =Co-translational targeting and translocation of the amino terminus of opsin across the endoplasmic membrane requires GTP but not ATP | journal =J. Biol. Chem. | year=2003 | volume=278 | issue=10 | pages=7920–7926 | pmid =12486130 | doi =10.1074/jbc.M207462200}}</ref><ref>{{cite journal | author =Kanner EM, Klein IK. ''et al''. | title =The amino terminus of opsin translocates "posttranslationally" as efficiently as cotranslationally | journal =Biochemistry | year=2002 | volume=41 | issue=24 | pages=7707–7715 | pmid =12056902 | doi =10.1021/bi0256882}}</ref> A great deal of the mechanics of transmembrane topology and folding remains to be elucidated.
==Sorting of proteins to mitochondria==
Most [[mitochondrion|mitochondrial]] [[protein]]s are synthesized as [[cytosolic]] precursors containing uptake [[peptide signal]]s. [[Cytosolic]] [[chaperone (protein)|chaperone]]s deliver [[preprotein]]s to channel linked receptors in the [[mitochondrial membrane]]. The [[preprotein]] with presequence targeted for the [[mitochondria]] is bound by [[Receptor (biochemistry)|receptors]] and the General Import Pore (GIP) (Receptors and GIP are collectively known as Translocase of Outer Membrane or TOM) at the [[outer membrane]]. The preprotein is translocated through TOM as hairpin loops. The preprotein is transported through the [[intermembrane space]] by small TIMs (which also acts as molecular [[chaperone (protein)|chaperone]]s) to the TIM23 or 22 (Translocase of Inner Membrane) at the [[inner membrane]]. Within the [[matrix (biology)|matrix]] the [[targeting sequence]] is cleaved off by mtHsp70.
Three [[mitochondrial]] outer membrane [[Receptor (biochemistry)|receptors]] are known: TOM20, TOM22 and TOM70<br />
TOM70: Binds to internal targeting peptides and acts as a docking point for cytosolic chaperones.<br />
TOM20: Binds presequences<br />
TOM22: Binds both presequences and internal targeting peptides<br />
The TOM channel is a [[cation]] specific high conductance channel with a [[molecular weight]] of 410[[kDa]] and a pore [[diameter]] of 21Å.
The presequence translocase23 (TIM23) is localized to the mitochondial [[inner membrane]] and acts a pore forming protein which binds precursor proteins with its [[N-terminal]]. TIM23 acts a translocator for preproteins for the mitochondrial matrix, the inner mitochondrial membrane as well as for the intermembrane space. TIM50 is bound to TIM23 at the inner mitocondrial side and found to bind presequences. TIM44 is bound on the matrix side and found binding to mtHsp70. <br />
The presequence translocase22 (TIM22) binds preproteins exclusively bound for the inner mitochondrial membrane.
[[Mitochondrial matrix]] targeting sequences are rich in positively charged amino acids and hydroxylated ones.
Proteins are targeted to submitochondrial compartments by multiple signals and several pathways.
Targeting to the outer membrane, [[intermembrane space]], and inner membrane often requires another signal sequence in addition to the matrix targeting sequence.
==Sorting of proteins to chloroplasts==
The preprotein for [[chloroplast]]s contain a stromal import sequence or a stromal and thylakoid targeting sequence. The majority of preproteins are translocated through the Toc and Tic complexes located within the chloroplast envelope. In the stroma the stromal import sequence is cleaved off and intra-chloroplast sorting and folding continues.
==Sorting of proteins to both chloroplasts and mitochondria==
Many proteins are needed in both [[mitochondria]] and [[chloroplasts]]. In general the targeting peptide is of intermediate character to the two specific ones. The targeting peptides of these [[proteins]] have a high content of basic and [[hydrophobic]] [[amino acids]], a low content of negatively charged [[amino acids]]. They have a lower content of alanine and a higher content of leucine and phenylalanine. The dual targeted proteins have a more hydrophobic targeting peptide than both mitochondrial and chloroplastic ones.
==Sorting of proteins to peroxisomes==
All [[peroxisome|peroxisomal]] proteins are encoded by nuclear genes.
To date there are two types of known Peroxisomal Targeting Signal (PTS):
'''Peroxisomal targeting signal 1 (PTS1)''': a C-terminal tripeptide with a consensus sequence (S/A/C)-(K/R/H)-(L/A). The most common PTS1 is [[serine]]-[[lysine]]-[[leucine]] ([[SKL]]). Most peroxisomal matrix proteins possess a PTS1 type signal.
'''Peroxisomal targeting signal 2 (PTS2)''': an N-terminal nonapeptide with a consensus sequence (R/K)-(L/V/I)-X5-(H/Q)-(L/A/F) (where X can be any amino acid).
There are also proteins that possess neither of these signals. Their transport is based on so-called "piggy-back" mechanism: such proteins associate with PTS1-possessing matrix proteins and are translocated into the peroxisomal matrix together with them.
===Diseases===
Peroxisomal protein transport is defective in the following genetic diseases:
*[[Zellweger syndrome]].
*[[Adrenoleukodystrophy]] (ALD).
*Refsum disease
==Receptor-mediated endocytosis==
Several molecules that attach to special receptors called [[coated pit]]s on the outside of cells cause the cell to perform [[endocytosis]], an invagination of the [[plasma membrane]] to incorporate the molecule and associated structures into [[endosomes]]. This mechanism is used for three main purposes:
*Uptake of essential metabolites, for example, [[LDL]].
*Uptake of some [[hormone]]s and [[growth factor]]s, for example, [[epidermal growth factor]] and [[nerve growth factor]].
*Uptake of proteins that are to be destroyed, for example, [[antigen]]s in [[phagocytosis|phagocytotic]] cells like [[macrophage]]s.
Receptor-mediated endocytosis can also be "abused":
*Some [[virus (biology)|virus]]es, for example, the [[Semliki forest virus]], enter the cell through this mechanism.
*[[Cholera]], [[diphtheria]], [[anthrax toxin|anthrax]], [[tetanus]], [[botulinum]], and other bacterial toxins enter the cell this way.
==Protein destruction==
Defective proteins are occasionally produced, or they may be damaged later, for example, by [[oxidation|oxidative]] stress. Damaged proteins can be recycled. Proteins can have very different [[half life]]s, mainly depending on their N-terminal amino acid residue. The recycling mechanism is mediated by [[ubiquitin]].
==Protein targeting in bacteria==
With some exceptions, [[Bacterium|Bacteria]] lack membrane-bound organelles as found in eukaryotes, but they may assemble proteins onto various types of inclusions such as gas vesicles and storage granules. Bacteria may have a single [[plasma membrane]] ([[Gram-positive bacteria]]), or both an inner (plasma) membrane and an outer [[cell wall]] membrane, with an aqueous space between the two called the [[periplasm]] ([[Gram-negative bacteria]]). Proteins may be incorporated into the plasma membrane, or either trapped in the periplasm or secreted into the environment, according to whether or not there is an outer membrane. The basic mechanism at the plasma membrane is similar to the eukaryotic one. In addition, bacteria may target proteins into or across the outer membrane. Systems for secreting proteins across the bacterial outer membrane may be quite complex and play key roles in pathogenesis. These systems may be described as type I secretion, type II secretion, etc.
In most Gram-positive bacteria, certain proteins are targeted for export across the plasma membrane and subsequent covalent attachment to the bacterial cell wall. A specialized enzyme, sortase, cleaves the target protein at a characteristic recognition site near the protein C-terminus, such as an LPXTG motif (where X can be any amino acid), then transfers the protein onto the cell wall. An system analogous to sortase/LPXTG, termed exosortase/PEP-CTERM, is proposed to exist in a broad range of Gram-negative bacteria.
==Secretory pathways==
The [[secretory pathway]] includes vesicular traffic, secretion, and endocytosis. [[Secretory protein]]s follow this pathway.
===Early stages===
Retrograde transport is common in the early stages. Proteins that have been successfully delivered to the [[Golgi apparatus]] advance through [[cisternal progression]].
===Later stages===
Coated vesicles mediate several transport steps.
==References==
<references/>
==See also==
* [[Bulk flow]]
* [[COPI]]
* [[COPII]]
* [[Clathrin]]
{{Protein topics}}
[[Category:Protein targeting]]
[[Category:Posttranslational modification]]
[[Category:Membrane proteins]]
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