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Protein targeting includes the mechanisms by which a biological cell transports proteins to the appropriate organelle for insertion into a membrane or secretion to the outside. (This article deals with protein targeting in eukaryotes except as noted.)1 Protein translocation
In 1970, Günter Blobel conducted experiments on the translocation of proteins across membranes. He was awarded the 1999 Nobel prize for his findings. He discovered that many proteins have a signal sequence, that is, a short amino acid sequence at one end that functions like a postal code for the target organelle. The translation of mRNA into protein by a ribosomeA ribosome is an organelle composed of rRNA (synthesized in the nucleolus) and ribosomal proteins. It translates mRNA into a polypeptide chain (e. a protein). It can be thought of as a factory that builds a protein from a set of genetic instructions. takes place within the cytosolThe cytosol (as opposed to cytoplasm which also includes the organelles) is the internal fluid of the cell, and a large part of cell metabolism occurs here. Proteins within the cytosol play an important role in signal transduction pathways, glycolysis, an. If the synthesized proteins "belong" in a different organelle, they can be transported there in either of two ways, depending on the protein.
1.1 Cotranslational translocation
The N-terminal signal sequence of the protein is recognized by a signal recognition particleThe signal recognition particle (SRP) is a protein- RNA complex that recognizes and transports specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes. The core of the particle is universal, being conserved in a (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 reticulumThe endoplasmic reticulum or ER (endoplasmic means "within the cytoplasm", reticulum means "little net") is an organelle found in all eukaryotic cells. The ER modifies proteins, makes macromolecules, and transfers substances throughout the cell. Prokaryot (ER, which is a membrane-bound organelle). There, the nascent protein is inserted into a protein channel that passes through the ER membrane. Within the ER, the protein is first covered by a chaperone proteinFor the person who accompanies another during social situations, see chaperon. In biology, chaperones are proteins whose function is to assist other proteins in achieving proper folding. They have been discovered in being heat shock proteins, that is, pro to protect it from the high concentration of other proteins in the ER, giving it time to foldProtein folding is the process by which a protein assumes its functional shape or conformation. All protein molecules are simple unbranched chains of amino acids, but it is by coiling into a specific three-dimensional shape that they are able to perform t correctly. Once folded, the protein is modified as needed (for example, by glycosylationGlycosylation is the addition of polysaccharides to molecules such as proteins. This process is the first of four principal modification steps in the synthesis of membrane proteins and secretory proteins. The majority of proteins synthesized in the rough), then transported into the Golgi apparatusIn cell biology, the Golgi apparatus Golgi complex or dictyosome is an organelle found in nearly all eukaryotic cells. The name comes from Italian anatomist Camillo Golgi, who identified it in 1898. Its primary function is to process proteins targeted to for further processing and sorting. From there, it goes to its target organelle. Upon translocation into that organelle, the signal sequence is removed.
