Figure 224

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. x38,000.

scribed from DNA to mRNA. Transcription is followed by translation, in which the coded message contained in the mRNA is "read" to form a polypeptide. A typical single molecule of mRNA may bind to many ribosomes spaced as close as 80 nucleotides apart, thus forming a polyribosome complex, or polysome. A polysome can translate a single mRNA molecule and simultaneously produce many copies of a particular protein.

Polysomes of the rER synthesize proteins for export from the cell and integral proteins of the plasma membrane

As polypeptide chains are synthesized by the membrane-bound polysomes, the protein is injected into the lumen of the cisterna, where it may be further modified, concentrated, or carried to another part of the cell in the continuous channels of the rER. The rER is particularly well developed in those cells that synthesize protein destined to leave the cell (secretory cells) as well as in cells with large amounts of plasma membrane, such as neurons. Secretory cells include glandular cells, fibroblasts, plasma cells, odontoblasts, ameloblasts, and osteoblasts. The rER is not limited, however, to secretory cells and neurons. Virtually every cell of the body contains profiles of rER. However, they may be few in number, a reflection of the amount of protein secretion, and dispersed so that in the light microscope they are not evident as areas of basophilia.

In agreement with the observation that the rER is most highly developed in active secretory cells, secretory proteins are synthesized exclusively by the ribosomes of the rER. In all cells, however, the ribosomes of the rER synthesize proteins that are to become permanent components of the lysosome, Golgi apparatus, rER, or nuclear envelope (these structures are discussed below) or integral components of the plasma membrane.

Signal peptides are attached to secretory proteins and integral proteins of the plasma membrane

If the protein to be synthesized is destined for export or will become part of the plasma membrane, the first group of amino acids that are linked to one another form a hydrophobic signal peptide (signal sequence) that binds to a receptor on the membrane of the rER (Fig. 2.25). When the ribosome (polysome) binds to the rER membrane, the signal peptide or a subsequent sequence instructs the newly formed peptide to pass through the membrane into the lumen of the rER cisterna. For simple secretory proteins, the polypeptide continues to be inserted into the lumen as it is synthesized. For integral membrane proteins, sequences along the polypeptide may instruct the forming protein to pass back and forth through the membrane, creating the functional domains that the protein will exhibit at its final membrane location.

P site A site

mRNA 5

ribosome dissociates rER lumen ribosome receptor membrane pore signal sequence removed protein

P site A site signal sequence threaded through membrane pore protein threaded through pore rER lumen ribosome receptor membrane pore signal sequence removed protein mRNA 5

ribosome dissociates

Secretory proteins pass through the membrane of the rER to its lumen, where they are modified and stored

The hydrophobic signal domain of a forming secretory protein attaches to a receptor on the membrane of the rER; as synthesis proceeds, the protein is inserted into and through the membrane. This process is described as con-translational insertion of protein into the rER. If the forming protein is not to be threaded in its entirety through the membrane, a new hydrophobic signal domain stops the threading process, permanently anchoring the protein in the membrane at this site.

On completion of protein synthesis, the ribosome detaches from the rER membrane and is again free in the cytoplasm. The region of the newly formed protein that extends into the lumen of the rER is modified by enzymes

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