Protein sorting or protein trafficking using signal proteins
The proteins synthesized in the cell have to be translocated to appropriate location within the same cell or other cells. This process is called protein sorting or protein trafficking and involves interaction of celt membranes either with some parts of newly synthesized protein or with a protein sequence added to this new protein. These protein sequences interacting with membranes are called signal sequences or transit peptides, or leader sequences, which are recognized by receptors located within the membrane. The transfer of proteins may be coupled with translation- co-translational transfer (proteins synthesized on ribosomes attached to endoplasmic reticulum = rough ER) or may take place after the translation- post-translational transfer (proteins synthesized on free ribosomes).
Proteins that are imported into mitochondria and chloroplasts possess N-terminal leader sequences that target them to organelle envelope or to inside the organelle. The leader sequence may have regions, which comprise envelope targeting signal and matrix targeting signal. For instance, cytochrome C, which is bound on inner membrane of mitochondria facing the intermembrane space has a leader sequence of 61 amino acids, of which 32 N-terminal amino, acids (mainly charged) make matrix targeting signal and the next 19 amino acids (uncharged) make envelope targeting signal. These regions of leader sequence decide the ultimate location of protein. Eventually, the leader sequence is cleaved by a protease and degraded. The entire process requires ATP as a source of energy.
The proteins synthesized on rough ER are transported, while still being synthesized. A signal hypothesis has been proposed for this transport. According to this hypothesis ribosomes synthesizing proteins are attached to the membrane via the leader sequence. In the protein, the presence of a signal recognition particle (SRP) in the form of a ribonucleoprotein complex (IIS), and in the membrane, the presence of a SRP receptor help in the transfer of a ribosome (engaged in protein synthesis) to the membrane. The function of SRP and, SRP receptor, is now over and transport of protein is facilitated by membrane bound ribosomes.
Several other mechanisms for protein transport or protein trafficking have been discovered. The function of signal sequences has been confirmed in many cases by joining a specific signal sequence to an unrelated protein (using a hybrid gene). In these cases the signal sequence directs the transport of unrelated proteins to cell organelles. For more details, readers are advised to consult Lewin's "Genes . V".
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