Ribosomes are small electron-dense particles, about 20 x 30 nm in size. They are composed of four types of rRNA and almost 80 different proteins.
There are two classes of ribosomes. One class is found in prokaryotes, chloroplasts, and mitochondria; the other is found in eukaryotic cells. Both classes of ribosomes are composed of two different-sized subunits.
In eukaryotic cells, the RNA molecules of both subunits are synthesized within the nucleus. Their numerous proteins are synthesized in the cytoplasm and then enter the nucleus and associate with rRNAs. Subunits then leave the nucleus, via nuclear pores, to enter the cytoplasm and participate in protein synthesis.
Ribosomes are intensely basophilic because of the presence of numerous phosphate groups of the constituent rRNA that act as polyanions. Thus, sites in the cytoplasm that are rich in ribosomes stain intensely with basic dyes, such as methylene and toluidine blue. These basophilic sites also stain with hematoxylin.
The individual ribosomes (Figure 2–15A) are held together by a strand of mRNA to form polyribosomes (polysomes). The message carried by mRNA is a code for the amino acid sequence of proteins being synthesized by the cell, and the ribosomes play a crucial role in decoding, or translating, this message during protein synthesis. Proteins synthesized for use within the cell and destined to remain in the cytosol (eg, hemoglobin in immature erythrocytes) are synthesized on polyribosomes existing as isolated clusters within the cytoplasm. Polyribosomes that are attached to the membranes of the endoplasmic reticulum (via their large subunits) translate mRNAs that code for proteins that are segregated into the cisternae of the reticulum (Figure 2–15B). These proteins can be secreted (eg, pancreatic and salivary enzymes) or stored in the cell (eg, enzymes of lysosomes, proteins within granules of white blood cells [leukocytes]). In addition, integral proteins of the plasma membrane are synthesized on polyribosomes attached to membranes of the endoplasmic reticulum (Figure 2–6).
Diagram illustrating (A) the concept that cells synthesizing proteins (represented here by spirals) that are to remain within the cytoplasm possess (free) polyribosomes (ie, nonadherent to the endoplasmic reticulum). In B, where the proteins are segregated in the endoplasmic reticulum and may eventually be extruded from the cytoplasm (export proteins), not only do the polyribosomes adhere to the membranes of rough endoplasmic reticulum, but the proteins produced by them are injected into the interior of the organelle across its membrane. In this way, the proteins, especially enzymes such as ribonucleases and proteases, which could have undesirable effects on the cytoplasm, are separated from it.
The cytoplasm of eukaryotic cells contains an anastomosing network of intercommunicating channels and sacs formed by a continuous membrane, which encloses a space called a cisterna. In sections, cisternae appear separated, but high-resolution microscopy of whole cells reveals that they are continuous. This membrane system is called the endoplasmic reticulum (Figure 2–16). In many places the cytosolic side of the membrane is covered by polyribosomes synthesizing protein molecules, which are injected into the cisternae. This permits the distinction between the two types of endoplasmic reticulum: rough and smooth
The endoplasmic reticulum is an anastomosing network of intercommunicating channels and sacs formed by a continuous membrane. Note that the smooth endoplasmic reticulum (foreground) is devoid of ribosomes, the small dark dots that are present in the rough endoplasmic reticulum (background). The cisternae of the smooth reticulum are tubular, whereas in the rough reticulum they are flat sacs.
Rough Endoplasmic Reticulum
Rough endoplasmic reticulum (RER) is prominent in cells specialized for protein secretion, such as pancreatic acinar cells (digestive enzymes), fibroblasts (collagen), and plasma cells (immunoglobulins). The RER consists of saclike as well as parallel stacks of flattened cisternae (Figure 2–13), limited by membranes that are continuous with the outer membrane of the nuclear envelope. The name "rough endoplasmic reticulum" alludes to the presence of polyribosomes on the cytosolic surface of this structure's membrane (Figures 2–16 and 2–17). The presence of polyribosomes also confers basophilic staining properties on this organelle when viewed with the light microscope.
Schematic representation of a small portion of the rough endoplasmic reticulum to show the shape of its cisternae and the presence of numerous ribosomes that are part of polyribosomes. It should be kept in mind that the cisternae appear separated in sections made for electron microscopy, but they form a continuous tunnel in the cytoplasm.
The principal function of the RER is to segregate proteins not destined for the cytosol. Additional functions include the initial (core) glycosylation of glycoproteins, the synthesis of phospholipids, the assembly of multichain proteins, and certain posttranslational modifications of newly formed polypeptides.
All protein synthesis begins on polyribosomes that are not attached to the endoplasmic reticulum. mRNAs of proteins destined to be segregated in the endoplasmic reticulum contain an additional sequence of bases at their 5´ end that codes for approximately 20–25 mainly hydrophobic amino acids called the signal sequence. Upon translation, the signal sequence interacts with a complex of six nonidentical polypeptides plus a 7S RNA molecule that is referred to as the signal-recognition particle (SRP). The SRP inhibits further polypeptide elongation until the SRP–polyribosome complex binds to a receptor in the membrane of the RER, the docking protein. Upon binding to the docking protein, the SRP is released from the polyribosomes, allowing the translation to continue (Figure 2–18).
The transport of proteins across the membrane of the rough endoplasmic reticulum (RER). The ribosomes bind to mRNA, and the signal peptide is initially bound to a signal-recognition particle (SRP). Ribosomes bind to the RER by interacting with the SRP and a ribosomal receptor. The signal peptide is then removed by a signal peptidase (not shown). These interactions cause the opening of a pore through which the protein is extruded into the RER.
Once inside the lumen of the RER, a specific enzyme, signal peptidase, located at the inner surface of the RER removes the signal sequence. Translation of the protein continues, accompanied by intracisternal secondary and tertiary structural changes as well as certain posttranslational modifications such as hydroxylation, glycosylation, sulfating, and phosphorylation.
Proteins synthesized in the RER can have several destinations: intracellular storage (eg, in lysosomes and specific granules of leukocytes), provisional intracellular storage of proteins for export (eg, in the pancreas, some endocrine cells), and as a component of other membranes (eg, integral proteins). Figure 2–19 shows several cell types with clear differences in the destination of the proteins they synthesize.
The ultrastructure of a cell that synthesizes (but does not secrete) proteins on free polyribosomes (A); a cell that synthesizes, segregates, and stores proteins in organelles (B); a cell that synthesizes, segregates, and directly exports proteins (C); and a cell that synthesizes, segregates, stores in supranuclear granules, and exports proteins (D).
Smooth Endoplasmic Reticulum
Smooth endoplasmic reticulum (SER) also takes the form of a membranous network within the cell; however, its ultrastructure differs from that of RER in two ways. First, SER lacks the associated polyribosomes that characterize RER. SER membranes therefore appear smooth rather than granular. Second, its cisternae are more tubular and more likely to appear as a profusion of interconnected channels of various shapes and sizes than as stacks of flattened cisternae (Figures 2–16 and 4–36). SER is continuous with the RER (Figure 2–16).
SER is associated with a variety of specialized functional capabilities. In cells that synthesize steroid hormones (eg, cells of the adrenal cortex), SER occupies a large portion of the cytoplasm and contains some of the enzymes required for steroid synthesis (Figure 4–36). SER is abundant in liver cells, where it is responsible for the oxidation, conjugation, and methylation processes employed by the liver to degrade certain hormones and neutralize noxious substances such as barbiturates. Another important function of SER is the synthesis of phospholipids for all cell membranes. The phospholipid molecules are transferred from the SER to other membranes (1) by vesicles that detach and are moved along cytoskeletal elements by the action of motor proteins, (2) through direct communication with the RER, or (3) by transfer proteins (Figure 2–20). SER contains the enzyme glucose-6-phosphatase, which is involved in the utilization of glucose originating from glycogen in liver cells. This enzyme is also found in RER, an example of the lack of absolute partitioning of functions between these organelles. SER participates in the contraction process in muscle cells, where it appears in a specialized form, called the sarcoplasmic reticulum, that is involved in the sequestration and release of the calcium ions that regulate muscular contraction (see Chapter 10: Muscle Tissue).
Schematic representation of a phospholipid-transporting amphipathic protein. Phospholipid molecules are transported from lipid-rich (SER) to lipid-poor membranes. (Redrawn and reproduced, with permission, from Junqueira LC, Carneiro J: Biologia Celular e Molecular, 6th ed. Editora Guanabara, 1997.)
|Afzelius BA, Eliasson R: Flagellar mutants in man: on the heterogeneity of the immotile-cilia syndrome. J Ultrastruct Res 1979;69:43. [PMID: 501788] |
|Aridor M, Balch WE: Integration of endoplasmic reticulum signaling in health and disease. Nat Med 1999;5:745. [PMID: 10395318] |
|Barrit GJ: Communication Within Animal Cells. Oxford University Press, 1992. |
|Becker WM et al: The World of the Cell, 4th ed. Benjamin/Cummings, 2000. |
|Bretscher MS: The molecules of the cell membrane. Sci Am 1985;253:100. [PMID: 2416050] |
|Brinkley BR: Microtubule organizing centers. Annu Rev Cell Biol 1985;1:145. [PMID: 3916316] |
|Brown MS et al: Recycling receptors: the round-trip itinerary of migrant membrane proteins. Cell 1983;32:663. [PMID: 6299572] |
|Cooper GM: The Cell: A Molecular Approach. ASM Press/Sinauer Associates, Inc., 1997. |
|DeDuve C: A Guided Tour of the Living Cell. Freeman, 1984. |
|DeDuve C: Microbodies in the living cell. Sci Am 1983;248:74. |
|Dustin P: Microtubules, 2nd ed. Springer-Verlag, 1984. |
|Farquhar MG: Progress in unraveling pathways of Golgi traffic. Annu Rev Cell Biol 1985;1:447. [PMID: 3916320] |
|Fawcett D: The Cell, 2nd ed. Saunders, 1981. |
|Krstíc RV: Ultrastructure of the Mammalian Cell. Springer-Verlag, 1979. |
|Mitchison TJ, Cramer LP: Actin-based cell motility and cell locomotion. Cell 1996;84:371. [PMID: 8608590] |
|Osborn M, Weber K: Intermediate filaments: cell-type-specific markers in differentiation and pathology. Cell 1982;31:303. [PMID: 6891619] |
|Pfeffer SR, Rothman JE: Biosynthetic protein transport and sorting in the endoplasmic reticulum. Annu Rev Biochem 1987;56:829. [PMID: 3304148] |
|Rothman J: The compartmental organization of the Golgi apparatus. Sci Am 1985;253:74. [PMID: 3929377] |
|Simons K, Ikonen E: How cells handle cholesterol. Science 2000;290:1721. [PMID: 11099405] |
|Tzagoloff A: Mitochondria. Plenum, 1982. |
|Weber K, Osborn M: The molecules of the cell matrix. Sci Am 1985;253:110. [PMID: 4071030]|