Actin Microfilament network

The transformation between sol and gel states results from the disassembly and reassembly of actin microfilament networks in the cytosol. Several actin-binding proteins probably control this process and hence the viscosity of the cytosol. Profilln at the front of the cell promotes actin polymerization, and a-actinin and filamin form gel-llke actin networks In the more viscous ectoplasm, as discussed earlier. Conversely, proteins such as cofilin sever actin filaments to form the more fluid endoplasm. 

This spectrin network further binds to actin microfilaments and to numerous other ligands. These associations are probably dynamic. For example, phosphorylation of ankyrin can alter its affinity for spectrin. The functions of the multiple protein-interaction domains of both spectrin and ankyrin have been as yet only partially defined (see Ch. 8). 

Pollard TD, Almo S, Quirk S etal. (1994) Structure of actin binding proteins Insights about function at atomic resolution. In Annu. Rev. Cell Biol. 10 207-49 Popoff MR, Rubin EJ, Gill DM et al. (1988) Actin-specific ADP-ribosyltransferase produced by a Clostridium difficile strain. In Infect. Immun. 56 2299-306 Popoff MR, Boquet P (1988) Clostridium spiroforme toxin is a binary toxin which ADP-ribosylates cellular actin. In Biochem. Biophys. Res. Commun. 152 1361—8 Reuner KH, Presek P, Boschek CB et al. (1987) Botulinum C2 toxin ADP-ribosylates actin and disorganizes the microfilament network in intact cells. In Eur. J. Cell Biol. 43 134-40... 

Reuner KH, Presek P, Boschek OB, etal. (1987) Botulinum 02 toxin ADP-ribosylates actin and disorganizes the microfilament network in intact cells. In European Journal of Cell Biology. 43 134-40... 

C. botulinum C2 toxin and C perfringens iota toxin belong to the family of actin-ADP-ribosylating toxins that transfer ADP-ribose from NAD to arginine-177 of actin. This modification results in inhibition of actin polymerization, leading to depolymerization of the microfilament network. Origin, structure, molecular mechanisms and general aspects of the use of this family of toxins is described in chapter 8, 9 and 10. 

Actin polymers form the thin filaments (also called microfilaments) in the cell that are organized into compact ordered bundles or loose network arrays by cross-linking proteins. Short actin filaments bind to the cross-linking protein spectrin to form the cortical actin skeleton network (see Fig. 10.6). In muscle cells, long actin fdaments combine with thick filaments, composed of the protein myosin, to produce muscle contraction. The assembly of G-actin subunits into polymers, bundling of fibers, and attachments of actin to spectrin and to the plasma membrane proteins and organelles, are mediated by a number of actin-binding proteins and G-proteins from the Rho family. 

The existence of profilin II, the isoform preferentially expressed in the mammalian neuronal system, has received much attention. Like other morphogenetic cellular processes, formation and maintenance of excitatory and inhibitory synapses depend on the actin cy-toskeleton. Neurotransmitter release by vesicle exoc)4osis at the presynaptic side as well as receptor clustering and activation at the posts)maptic side both depend on microfilament networks (reviewed in Jockusch et al. 2004), and enforcement of specific synaptic connections, which are the basis for brain-specific tasks such as memory, require morphological shape... 

The ability of lipopolysaccharide to cause altered phosphate labelling of p/y-actin suggests a participation of the microfilament network in lipopoly-saccharide-induced monocyte activation (Haus-CHiLDT et al. 1997). 

Blood platelets are key players in the blood-clotting mechanism. These tiny fragments of cytoplasm are shed into the circulation from the surface of megakaryocytes located in the bone marrow. When the lining of a blood vessel is injured, activated platelets release clotting factors, adhere to each other and to damaged surfaces, and send out numerous filopodia. The shape changes that occur in activated platelets are the result of actin polymerization. Before activation, there are no microfilaments because profilin binds to G-actin and prevents its polymerization. After activation, profilin dissociates from G-actin, and bundles and networks of F-actin filaments rapidly appear within the platelet. 

Nonmuscle cells perform mechanical work, including self-propulsion, morphogenesis, cleavage, endocytosis, exocytosis, intracellular transport, and changing cell shape. These cellular functions are carried out by an extensive intracellular network of filamentous structures constimting the cytoskeleton. The cell cytoplasm is not a sac of fluid, as once thought. Essentially all eukaryotic cells contain three types of filamentous struc-mres actin filaments (7-9.5 nm in diameter also known as microfilaments), microtubules (25 nm), and intermediate filaments (10-12 nm). Each type of filament can be distinguished biochemically and by the electron microscope. 

Microfilaments of F actin traverse the microvilli in ordered bundles. The microfila-ments are attached to each other by actin-as-sociated proteins, particularly fimbrin and vil-lin. Calmodulin and a myosin-like ATPase connect the microfilaments laterally to the plasma membrane. Fodrin, another microfila-ment-associated protein, anchors the actin fibers to each other at the base, as well as attaching them to the cytoplasmic membrane and to a network of intermediate filaments. In this example, the microfilaments have a mainly static function. In other cases, actin is also involved in dynamic processes. These include muscle contraction (see p. 332), cell movement, phagocytosis by immune cells, the formation of microspikes and lamellipo-dia (cellular extensions), and the acrosomal process during the fusion of sperm with the egg cell. 

The cytoskeletons of other eukaryotic cells typically include both microtubules and microfilaments, which consist of long, chainlike oligomers of the proteins tubulin and actin, respectively. Bundles of microfilaments often lie just underneath the plasma membrane (fig. 17.22). They participate in processes that require changes in the shape of the cell, such as locomotion and phagocytosis. In some cells, cytoskeletal microfilaments appear to be linked indirectly through the plasma membrane to peripheral proteins on the outer surface of the cell (fig. 17.23). Among the cell surface proteins connected to this network is fibronectin, a glycoprotein believed to play a role in cell-cell interactions. The lateral diffusion of fibronectin is at least 5,000 times slower than that of freely diffusible membrane proteins. 

Thread-like actin filaments (microfilaments) spread through the hepatocytes creating a three-dimensional network and ensuring both form and stability of the cell. They also guarantee the shape of the microvilli and fenestrae as well as supporting the mechanical functions of the canaliculi. In addition, they influence the viscosity of the cytoplasm. Cytochalasin A depolymerizes... 

Cortical spectrin-actin networks are attached to the cell membrane by bivalent membrane-microfilament binding proteins such as ankyrin and band 4.1 (see Figure 5-31). 

As mentioned previously, the actin cytoskeleton is not a static, unchanging structure consisting of bundles and networks of filaments. Rather, the microfilaments in a cell are constantly shrinking or growing in length, and bundles and meshworks... 

With respect to the mechanical properties of the plasma membrane the microfilament system is considered the most important since a network of these filaments imderhes the plasma membrane and stabilizes it. This membrane supporting network of actin filaments is often called the cortical actin. In order to test whether the mechanical properties of the membrane (de-... 

---