Protein targeting

The post-translational protein transport in biology denotes a mechanism of the proteins according to their synthesis (see translation) is transported through a membrane. Different forms of post-translational protein transport can be found in pro- and eukaryotic cells. Find the (partial ) Transport of proteins still during translation instead, it is called cotranslationalem transport.

Properties

Posttranslational protein transport takes place mainly in organelles such as mitochondria, plastids (and also at the thylakoids ) and peroxisomes and the nucleus. In addition, one finds posttranslational protein transport to the cytoplasmic membrane of bacteria.

Another form of post-translational transport can be found in the endoplasmic reticulum (ER) where proteins of the cytosolic side of the ER membrane is transported into the lumen of the ER.

In this process, the proteins are completely synthesized in the cytoplasm and will then be transported through the ER membrane. The majority of the currently available information on such shape of the ER transport comes from studies that were performed on the yeast Saccharomyces cerevisiae. The signal sequence of a protein determines whether a secretory protein is co- or post-translationally transported into the ER. Wherein hydrophilic signal sequences the affinity of the signal recognition particle (SRP ) for the signal sequence appears to be less pronounced, resulting in any solid bonding and thus no translation pause which would initiate the co-translational transport. The protein is completely synthesized in the cytoplasm and released from the ribosome.

To keep the transport substrate in a translokationskompetenten state cytosolic chaperones bind the Hsp70 family to the protein. Little is known about the subsequent process of the target line of the protein to the ER membrane. The bound cytosolic factors appear to play no essential role, however, for the actual transport across the ER membrane, are transported posttranslationally as by urea denatured (and therefore of HSP70 freed ) transport proteins in vitro as efficient as the native proteins.

Reconstitution with purified components from yeast microsomes showed that in addition to the luminal chaperone Kar2p and ATP, a special membrane complex is required to post-translationally transported prepro -a- factor in vitro. When this complex is the hetero - heptameric Sec complex, which is composed of the trimeric Sec61 complex and the tetrameric Sec62/Sec63-Komplex. SEC complex forms, similar to the ribosome by the Sec61 complex ring structures in the membrane of.

The actual post-translational transport of the proteins through the ER membrane is carried out in two steps. First binds to transporting protein, Kar2p - and ATP - independent of the Sec complex, the cytosolic components of the underlying Sec62/Sec63-Subkomplexes likely to form a kind of signal sequence antenna in the membrane. By cross-linking studies have indicated that the signal sequence of the protein during this period is recognized and bound by the large subunit of the Sec61 complex. In the second step of the transport process, the bound polypeptide is moved as in the co-translational transport through the channel formed by the Sec61 complex wherein additional Kar2p and ATP are required for efficient transport. When Kar2p is an Hsp70 homologous protein, which is required both in vivo and in vitro for the posttranslational transport. The ATP -bound form of Kar2p binds to the lying in the lumen of the ER DnaJ homologous domain of the Sec63p protein from the Sec62/63-Subkomplex and is transferred under ATP hydrolysis with low sequence specificity of the polypeptide chain of the protein to be translocated.

For the model protein prepro -a- factor has been shown in vitro that the transfer is performed by the ER- membrane by the principle of a molecular ratchet. The transport substrate can diffuse freely in both directions as a result of Brownian motion in Translokonskanal. Only with the addition of the luminal Kar2p to the polypeptide chain creates a directed transport process, as contained in the lumen areas of the polypeptide chain can not slide back into the channel through the bound Kar2p. Through the successive binding of other proteins Kar2p to the polypeptide chain of the transport of the protein occurs in the lumen of the ER.

Model of posttranslational protein transport across the ER membrane of the yeast Saccharomyces cerevisiae.

Bacterial protein secretion must overcome the outer membrane of bacteria in Gram-negative bacteria also. End, the organisms have ( up to type V type I) developed at least five different systems.

• Protein transport