| PROTEIN TARGETING AND POST-TRANSLATIONAL MODIFICATIONS
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| More than one ribosome can translate an mRNA at the same time. An mRNA with several bound ribosomes is known as a polyribosome or polysome (Fig. 32.8). There are two general classes of polysomes found in cells: those that are free in the cytoplasm, and those that are attached to the endoplasmic reticulum. mRNAs encoding proteins destined for the cytoplasm are translated primarily on polysomes free in the cytoplasm, while mRNAs encoding membrane and secretory proteins are translated on polysomes attached to the endoplasmic reticulum. Translation on the endoplasmic reticulum assists in the post-translational modification of proteins and targeting of proteins to specific subcellular compartments.
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| Cellular fate of proteins is determined by their signal peptide sequences
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Proteins that are destined for export, for insertion into membranes, or for specific cellular organelles, must in some way be distinguished from proteins that reside in the cytoplasm. The distinguishing characteristic of all proteins targeted for these locations is that they contain a signal sequence usually comprising the first 20-30 amino acids on the amino-terminal end of the protein. In the case of secretory or membrane proteins, shortly after the signal sequence is synthesized, it is recognized by a ribonucleoprotein complex known as a signal recognition particle (SRP). The SRP binds to the signal sequence and halts translation of the remainder of the protein. This complex then binds to a receptor, known as the SRP docking protein, located on the membrane of the endoplasmic reticulum. After the SRP has delivered the ribosome-bound mRNA with its nascent protein to the endoplasmic reticulum, the signal sequence is inserted through the membrane, the SRP dissociates, and translation continues with the polypeptide chain being moved, as it is synthesized, across the membrane into the interstitial space of the endoplasmic reticulum (Fig. 32.9). The protein is then transferred to the Golgi apparatus, and then to its final destination.
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| Figure 32.8 Protein synthesis on polysomes. Protein can be synthesized by several ribosomes bound to the same mRNA, forming a structure known as a polysome. |
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| Figure 32.9 Protein synthesis on endoplasmic reticulum. Proteins can be targeted to specific sites in the cell by recognition of the signal sequence by an SRP. This complex is then recognized by the SRP docking protein (SRP receptor) on the endoplasmic reticulum (ER), where the signal sequence is inserted through the membrane. A signal peptidase removes the signal sequence and translation resumes. Once synthesis of the protein is complete, the protein is delivered to its cellular location. |
| In contrast to secretory and membrane proteins, mitochondrial and nuclear proteins are transported after their translation is complete. In common with membrane and secretory proteins, both mitochondrial and nuclear proteins have signal sequences that mark them to be transported from
the endoplasmic reticulum to their respective organelles. In the case of proteins destined for the mitochondrion, they may have two signal sequences on their amino-terminal ends, depending on whether they are destined for the matrix or for the intermembrane space. Mitochondrial proteins must be unfolded before they can be transported; in contrast, nuclear proteins may have nuclear localization signals throughout the entire length of the molecule and do not have to be unfolded before transport. Nuclear pores are very large complexes and apparently accommodate the transport of a protein in its native state.
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