This also helps in identifying novel components for particular specific pathways and components common for all pathways. Mitochondrial proteins tend to be nuclear encoded and these proteins must be translocated to the compartments within the mitochondria (Copley, 1997; Berg 2002). The final localisation is determined with interaction of nascent polypeptides with inner and outer membrane translocation complexes.
Protein synthesis tends to occur in cytosols and in mitochondria, chloroplasts and the location of proteins and crossing of proteins across plasma membrane could be studied using the concept of protein translocation. This essay is based on the discussion of protein translocation and the mechanism involved in such translocation including the reasons why proteins seem to cross membranes and whether it is done as chains or after folding.
The translocation of proteins was first experimentally discovered by Blobel in 1970 and he discovered that proteins tend to have signal sequence or a short amino acid sequence and this functions as a code address for the target organelle. mRNA translates into protein with the help of a ribosome and this process of translation takes place within the cytosol (Agarraberes and Dice, 2001, Berg, 2002). When synthesized proteins belong to specific organelles they can be transported through cotranslational translocation or posttranslational translocation. In the cotranslational translocation, the N terminal signal sequence is recognised by a signal recognition particle or SRP at the time when the protein is being synthesized on the ribosome (Agarraberes and Dice, 2001; Berg, 2002). The ribosome protein complex is transferred to the SRP receptor on the endoplasmic reticulum and the nascent protein is inserted in the translocation complex and passes through the ER membrane. The signal sequence is translocated from the polypeptide into the ER by signal peptidase although signal sequence processing differs for some ER transmembrane proteins (Pohlschroder et al, 2004). In the ER, the protein is covered by a chaperone protein that tends to protect it from the high concentration of other proteins in the ER and the protein goes through its folding mechanism. After the folding takes place, protein modification occurs and the protein is then transported to the golgi apparatus for further processing or goes to target organelles or even retained in ER by ER retention mechanisms.
Diagrammatic representation of the Protein Translocation process
For describing the process of translocation, it is necessary to understand the mechanisms about how proteins get to where they should be and how they specify the locations. The process involves targeting and identifying an organelle and moving or translocating along its membrane although most transport systems tend to be substrate specific. Wickner and Schekman (2005) examine how protein sequences are