Механизмы нацеливания и сортировки белков в клетке.

Biological mechanism for routing proteins
Protein targeting or protein sorting is the biological mechanism by which proteins are transported to their appropriate destinations within or outside the cell. This article deals with protein targeting in eukaryotes unless specified otherwise Proteins can be targeted to the inner space of an organelle, different intracellular membranes, the plasma membrane, or to the exterior of the cell via secretion. Correct sorting is crucial for the cell; errors or dysfunction in sorting have been linked to multiple diseases.

In 1970, Günter Blobel conducted experiments on protein translocation across membranes. Blobel, then an assistant professor at Rockefeller University, built upon the work of his colleague George Palade. Palade had previously demonstrated that non secreted proteins were translated by free ribosomes in the cytosol, while secreted proteins (and target proteins, in general) were translated by ribosomes bound to the endoplasmic reticulum.

Signal peptides serve as targeting signals, enabling cellular transport machinery to direct proteins to specific intracellular or extracellular locations. While no consensus sequence has been identified for signal peptides, many nonetheless possess a characteristic tripartite structure: Unlike signal peptides, signal patches are composed by amino acid residues that are discontinuous in the primary sequence but become functional when folding brings them together on the protein surface. Unlike most signal sequences, signal patches are not cleaved after sorting is complete. In addition to intrinsic signaling sequences, protein modifications like glycosylation can also induce targeting to specific intracellular or extracellular regions.

Since the translation of mRNA into protein by a ribosome takes place within the cytosol, proteins destined for secretion or a specific organelle must be translocated. This process can occur during translation, known as co translational translocation, or after translation is complete, known as post translational translocation.

Most secretory and membrane bound proteins are co translationally translocated. Proteins that reside in the endoplasmic reticulum (ER), golgi or endosomes also use the co translational translocation pathway. This process begins while the protein is being synthesized on the ribosome, when a signal recognition particle (SRP) recognizes an N terminal signal peptide of the nascent protein. Binding of the SRP temporarily pauses synthesis while the ribosome protein complex is transferred to an SRP receptor on the ER in eukaryotes, and the plasma membrane in prokaryotes. There, the nascent protein is inserted into the translocon, a membrane bound protein conducting channel composed of the Sec61 translocation complex in eukaryotes, and the homologous SecYEG complex in prokaryotes. In secretory proteins and type I transmembrane proteins, the signal sequence is immediately cleaved from the nascent polypeptide once it has been translocated into the membrane of the ER (eukaryotes) or plasma membrane (prokaryotes) by signal peptidase. The signal sequence of type II membrane proteins and some polytopic membrane proteins are not cleaved off and therefore are referred to as signal anchor sequences. Within the ER, the protein is first covered by a chaperone protein to protect it from the high concentration of other proteins in the ER, giving it time to fold correctly. Once folded, the protein is modified as needed (for example, by glycosylation), then transported to the Golgi for further processing and goes to its target organelles or is retained in the ER by various ER retention mechanisms. The amino acid chain of transmembrane proteins, which often are transmembrane receptors, passes through a membrane one or several times. These proteins are inserted into the membrane by translocation, until the process is interrupted by a stop transfer sequence, also called a membrane anchor or signal anchor sequence. These complex membrane proteins are currently characterized 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 this model. Seven transmembrane G protein coupled receptors (which represent about 5% of the genes in 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 translocation, which has been demonstrated with opsin with in vitro experiments, breaks the usual pattern of "co translational" translocation which has always held for mammalian proteins targeted to the ER. A great deal of the mechanics of transmembrane topology and folding remains to be elucidated.

Even though most secretory proteins are co translationally translocated, some are translated in the cytosol and later transported to the ER/plasma membrane by a post translational system. In prokaryotes this process requires certain cofactors such as SecA and SecB and is facilitated by Sec62 and Sec63, two membrane bound proteins. The Sec63 complex, which is embedded in the ER membrane, causes hydrolysis of ATP, allowing chaperone proteins to bind to an exposed peptide chain and slide the polypeptide into the ER lumen. Once in the lumen the polypeptide chain can be folded properly. This process only occurs in unfolded proteins located in the cytosol. In addition, proteins targeted to other cellular destinations, such as mitochondria, chloroplasts, or peroxisomes, use specialized post translational pathways. Proteins targeted for the nucleus are also translocated post translationally through the addition of a nuclear localization signal (NLS) that promotes passage through the nuclear envelope via nuclear pores.

While some proteins in the mitochondria originate from mitochondrial DNA within the organelle, most mitochondrial proteins are synthesized as cytosolic precursors containing uptake peptide signals. Unfolded proteins bound by cytosolic chaperone hsp70 that are targeted to the mitochondria may be localized to four different areas depending on their sequences. They may be targeted to the mitochondrial matrix, the outer membrane, the intermembrane space, or the inner membrane. Defects in any one or more of these processes has been linked to health and disease.

Proteins destined for the mitochondrial matrix have specific signal sequences at their beginning (N terminus) that consist of a string of 20 to 50 amino acids. These sequences are designed to interact with receptors that guide the proteins to their correct location inside the mitochondria. The sequences have a unique structure with clusters of water loving (hydrophilic) and water avoiding (hydrophobic) amino acids, giving them a dual nature known as amphipathic. These amphipathic sequences typically form a spiral shape (alpha helix) with the charged amino acids on one side and the hydrophobic ones on the opposite side. This structural feature is essential for the sequence to function correctly in directing proteins to the matrix. If mutations occur that mess with this dual nature, the protein often fails to reach its intended destination, although not all changes to the sequence have this effect. This indicates the importance of the amphipathic property for the protein to be correctly targeted to the mitochondrial matrix. In addition to the docking of internal sequences and cytosolic chaperones to TOM70. This is accompanied by the necessary release of the cytosolic chaperones that maintain an unfolded state prior to entering the mitochondria. As the polypeptide enters the matrix, the signal sequence is cleaved by a processing peptidase and the remaining sequences are bound by mitochondrial chaperones to await proper folding and activity. It is this negative potential inside the matrix that directs the positively charged regions of the targeting sequence into its desired location.

Targeting of mitochondrial proteins to the inner membrane may follow 3 different pathways depending upon their overall sequences, however, entry from the outer membrane remains the same using the import receptor complex TOM20/22 and TOM40 general import core. The second intermembrane space pathway does not utilize any inner membrane complexes and therefor does not contain a matrix targeting signal. Instead, it enters through the general import core TOM40 and is further modified in the intermembrane space to achieve its active conformation. TIM9/10 is an example of a protein that follows this pathway in order to be in the location it needs to be to assist in inner membrane targeting.

Outer membrane targeting simply involves the interaction of precursor proteins with the outer membrane translocase complexes that embeds it into the membrane via internal targeting sequences that are to form hydrophobic alpha helices or beta barrels that span the phospholipid bilayer. Proteins targeted to the envelope of chloroplasts usually lack cleavable sorting sequence and are laterally displaced via membrane sorting complexes. General import for the majority of preproteins requires translocation from the cytosol through the Toc and Tic complexes located within the chloroplast envelope. Where Toc is an abbreviation for the translocase of the outer chloroplast envelope and Tic is the translocase of the inner chloroplast envelope. There is a minimum of three proteins that make up the function of the Toc complex. Two of which, referred to as Toc159 and Toc34, are responsible for the docking of stromal import sequences and both contain GTPase activity. The third known as Toc 75, is the actual translocation channel that feeds the recognized preprotein by Toc159/34 into the chloroplast.

Targeting to the stroma requires the preprotein to have a stromal import sequence that is recognized by the Tic complex of the inner envelope upon being translocated from the outer envelope by the Toc complex. The Tic complex is composed of at least five different Tic proteins that are required to form the translocation channel across the inner envelope. Upon being delivered to the stroma, the stromal import sequence is cleaved off via a signal peptidase. This delivery process to the stroma is currently known to be driven by ATP hydrolysis via stromal HSP chaperones, instead of the transmembrane electrochemical gradient that is established in mitochondria to drive protein import. In general the dual 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. However, it is tedious to predict if a peptide is dual targeted or not based on its physio chemical characteristics.

As discussed above (see protein translocation), most prokaryotic membrane bound and secretory proteins are targeted to the plasma membrane by either a co translation pathway that uses bacterial SRP or a post translation pathway that requires SecA and SecB. At the plasma membrane, these two pathways deliver proteins to the SecYEG translocon for translocation. Bacteria may have a single plasma membrane (Gram positive bacteria), or an inner membrane plus an outer membrane separated by the periplasm (Gram negative bacteria). Besides the plasma membrane the majority of prokaryotes 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.

In gram negative bacteria proteins may be incorporated into the plasma membrane, the outer membrane, the periplasm or secreted into the environment. 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. Several analogous systems are found that likewise feature a signature motif on the extra cytoplasmic face, a C terminal transmembrane domain, and cluster of basic residues on the cytosolic face at the protein's extreme C terminus. The PEP CTERM/exosortase system, found in many Gram negative bacteria, seems to be related to extracellular polymeric substance production. The PGF CTERM/archaeosortase A system in archaea is related to S layer production. The GlyGly CTERM/rhombosortase system, found in the Shewanella, Vibrio, and a few other genera, seems involved in the release of proteases, nucleases, and other enzymes.

Minimotif Miner is a bioinformatics tool that searches protein sequence queries for a known protein targeting sequence motifs. Phobius predicts signal peptides based on a supplied primary sequence. SignalP predicts signal peptide cleavage sites. LOCtree predicts the subcellular localization of proteins.