Protein sorting in the cell


Protein Sorting in the cell

In prokaryotes and eukaryotes, some of proteins produced are placed in different cell compartments, such as the nucleus, mitochondrion, chloroplast and lysosome. The sorting of proteins to appropriate compartments is under genetic control. The specific “signal” or “leader” sequences on the proteins direct them to the correct organelles.  In 1975, Gunther Blobel ,B. Dobberstein and colleagues found that secreted proteins and other protein sorted by the Golgi initially contain extra amino acid at the amino terminal end. This work  has led to the signal hypothesis, which states that proteins sorted by the Golgi bind to the ER by an hydrophobic amino terminal extension to the membrane that is subsequently removed and degraded. Gunther Blobel won the noble prize for his work in physiology or medicine in 1999.

The signal sequence  of a protein designed for ER consist of about 15-to-30 N-terminal amino acids. When the signal sequence is translated and exposed on the ribosome surface, a cytoplasmic signal recognition particle [SRP] binds to the sequence and blocks further translation of them RNA until the complex reaches and binds to the ER.

The SRP binds to a docking protein in the ER membrane. After firm binding of the ribosome to the ER, SRP get released and translation continues. The growing polypeptide extends into cisternal space of ER. Once signal sequence reaches into cisternal space of ER it get removed from polypeptide by enzyme signal peptidase. The modification happens to the polypeptide by adding specific carbohydrates groups to form Glycoproteins which are transferred in vesicles to the Golgi apparatus. Proteins in vesicles migrate to cell surface and fuse with plasma membrane and release proteins to outside of cell.


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