polysome. A polysome attached to the cytoplasmic surface of the rER can translate a single mRNA molecule and simultaneously produce many copies of a particular protein. In contrast, free ribosomes reside within the cytoplasm. They are not associated with any intracellular membranes and are structurally and functionally identical to polysomes of the rER.
The differences between the structure of prokaryotic (bacterial) and eukaryotic ribosomes were exploited by researchers, who discovered chemical compounds (antibiotics) that bind to bacterial ribosomes, thereby destroying a bacterial infection without harming the cells of the infected individual. Several types of antibiotics, such as aminoglycosides (streptomycin), macrolides (erythromycin), lincosamides (clindamycin), tetracyclines, and chloramphenicol inhibit protein synthesis by binding to different portions of bacterial ribosomes.
Signal peptides direct the post-translational transport of a protein.
FIGURE 2.26 • Electron micrograph of the rER and polyribosome complexes. This image shows a small section of the rER adjacent to the nucleus sectioned in two planes. The reticulum has turned within the section. Thus, in the upper right and left, the membranes of the reticulum have been cut at a right angle to their surface. In the center, the reticulum has twisted and is shown as in an aerial view (from above the membrane). The large spiral cytoplasmic assemblies (arrows) are chains of ribosomes that form polyribosomes that are actively engaged in translation of the mRNA molecule. 38,000.
Most proteins that are synthesized for export or to become a part of specific organelles (such as the plasma membrane, mitochondrial matrix, endoplasmic reticulum, or nucleus) require sorting signals that direct proteins to their correct destinations. These signal sequences (signal peptides) are often found in the sequence of the first group of 15 to 60 amino acids on the amino-terminus of a newly synthesized protein. For instance, almost all proteins that are transported to the endoplasmic reticulum have a signal sequence consisting of five to ten hydrophobic amino acids on their amino-termini. The signal sequence of the nascent peptide interacts with a signal-recognition particle (SRP), which arrests further growth of the polypeptide chain. The complex containing the SRP–polyribosome complex with arrested polypeptide synthesis is then relocated toward the rER excision of introns release of translational arrest