BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

PART V. MOLECULAR PHYSIOLOGY

CHAPTER 34. MUSCLE CONTRACTION AND CELL MOTILITY

34.16. Cytochalasin and Phalloidin Inhibit Motility Coupled with Assembly and Disaggregation of Actin Filaments

Cytochalasin B, a fungal alkaloid, alters the shape of Eukaryotic Cells when added to them and inhibits many forms of motility. For instance, this alkaloid inhibits The formation of ruffled borders in fibroblasts, the outgrowth of axons from ganglia, Blood clot retraction by platelets, and the division of fertilized sea urchin eggs. Electron Cell/15.html">Microscopy data indicate that cytochalasin affects microfilaments, as they disappear in cells treated with this alkaloid. It has been found that cytochalasin prevents the assembly of Actin filaments by specifically interacting with one of the filament ends. This inhibitory effect of cytochalasin demonstrates the dynamic Nature of the microfilament Structure.

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The Significance of the continuous processes of microfilament assembly and disaggregation for cellular movement was also revealed through The Study of the MECHANISM OF ACTION of phalloidin. This toxic agent is present in the poisonous mushroom Amanita phalloides, which also contains α-amanitin, an RNA polymerase inhibitor (Section 29.18). Like cytochalasin, phalloidin blocks those forms of motility that involve microfilaments. Phalloidin binds to actin units within microfilaments and thereby prevents their depolymerization. Thus, phalloidin effectively locks microfilaments in place.

34.17. Microtubules Participate in Various Types of Cell Motility and Partially Form the Cytoskeleton

So far, we have examined The Role of microfilaments in various types of cell motility. Let us now turn to another class of fibrous elements, namely microtubules, which are present in virtually all eukaryotic cells and participate in both maintaining cell architecture and contractile activity (Fig. 34.29). Microtubules are hollow cylindrical structures formed from two similar types of subunits (each with a mass of 55 kDa) — α- and β-tubulin. The outer diameter of microtubules is about 240 А, which clearly distinguishes them from microfilaments (70 А in diameter) and Intermediate filaments (structures 100 А in diameter that serve as connecting elements). The rigid wall of a microtubule is formed from helically arranged, alternating α- and β-tubulin subunits (Fig. 34.30). The repeating structural unit is the αβ-dimer, and overall, the structure is formed from 13 protofilaments running parallel to the long axis of the microtubule.

Fig. 34.29. Immunofluorescence micrograph demonstrating the distribution of microtubules in a fibroblast

Fig. 34.30. Helical arrangement of tubulin subunits in a microtubule. A - schematic representation of a cross-section showing that the microtubule is formed by 13 protofilaments. B - surface lattice of α- and β-subunits

The assembly and disaggregation of microtubules occur at opposite ends. Colchicine, an alkaloid from autumn crocuses, blocks assembly and thereby inhibits cellular processes that rely on microtubules. For example, colchicine halts Cell Division at the metaphase stage because microtubules are essential for chromosome segregation. Colchicine also inhibits the directed movement of various particles within cells, which underlies the suppression of their secretory processes. For several centuries, colchicine has been used to treat acute Gout attacks.

34.18. Ciliary Beating and Flagellar Movement Are Driven by Dynein-Induced Microtubule Sliding

Microtubules are the primary components of Cilia and flagella in eukaryotes. These Organelles are found in many cells. Motile cilia, acting like oars, generate fluid flow parallel to The Cell surface. For instance, the coordinated movement of cilia lining the respiratory tract helps clear foreign particles. Flagella are essential for the propulsion of free cells, such as spermatozoa or Protozoa. As electron microscopic studies have shown, The structure of cilia and flagella is fundamentally the same in virtually all eukaryotes. It consists of a bundle of fibers called an axoneme, which is surrounded by a membrane that is an extension of The Plasma Membrane. The fibers within the axoneme are microtubules. Nine double microtubules are located in the periphery, and two single microtubules are in the center (Fig. 34.31). This frequently recurring structural motif is known as the 9 + 2 arrangement. The diameter of the two central microtubules is 240 А. Each of the nine peripheral pairs appears as a figure 8 in cross-section and measures 370 x 250 А (Fig. 34.32). One of the microtubules in the pair, subfibril A, is smaller and connects to the central Sheath of the cilium via a radial spoke. Two arms project from each subfibril A. In an individual cilium, all arms point in the same direction. Treatment of cilia with a detergent followed by a high-salt solution results in the removal of the outer membrane and the solubilization of an ATPase called dynein. As a result of this treatment, the peripherally located fibers retain their cylindrical paired Organization but lose their arms. When dynein is added back in an appropriate ionic environment, the arms are restored. Consequently, the dynein-containing arms of subfibril A possess ATPase activity.

Fig. 34.31. Electron micrograph of a cross-section of a flagellar axoneme. Nine outer double microtubules surround two single ones

Fig. 34.32. Schematic representation of the axoneme structure

How does the Cleavage of ATP by dynein cause ciliary and flagellar movement? As demonstrated by Peter Satir and Jan Gibbons, the peripheral double microtubules of the axoneme slide relative to one another, thereby causing the cilium to bend. The sliding itself is driven by the formation of dynein cross-bridges. Apparently, as ATP is hydrolyzed, the dynein arms of one microtubule pair move along the adjacent pair in a manner entirely analogous to how Myosin cross-bridges move along the actin filament in Skeletal Muscle. In the intact cilium, radial spokes counteract the sliding, and it is thanks to them that a local bend occurs rather than a contraction of the cilium.

In examinations of a group of patients with chronic pulmonary diseases, axonemes lacking dynein were discovered. In this case, as shown by Björn Afzelius, the respiratory epithelium cilia were immotile. Moreover, men with this genetic defect were sterile due to the immobility of their spermatozoa. In another, recently discovered case, the immobility of cilia and flagella was found to be caused by a defect in the radial spokes. These clinical observations confirm current models of the movement mechanism of these organelles.