The cytoskeleton

Structural studies have indicated that fibronectin is composed of at least three types of modules, internal amino acid sequence homologies, that have been found in several other protein sequences. Two other attachment glycoproteins have been described recently, and their properties are compared with fibronectin in Table 5.3. 

Shape Extended V shape Cross shape 3 short arms, 1 long arm Compact 

Collagen-binding Types I-V best III and I Type IV Type II 

Cell-binding Fibroblasts Epithelial and endothelial cells Chondrocytes 

Genetic errors in or damage to cytoskeletal components can play a central role in disease including many forms of hemolytic anemia and the Duchenne and Becker muscular dystrophies. Appropriate cell-cell contact (regulated through the cytoskeleton) appears to be critical in preventing cells from becoming cancerous and, indeed, many types of cancerous cell exhibit abnormal cytoskeletons. 

Biopolymers are often found as part of three-dimensional (3D) biological structures in combination with cross-linking proteins or other associated molecules. This ability to form macrostructures allows some protein filaments to act as structural materials for the cell. The most well-known example of structurally important biological filaments is the cytoskeleton of the eukaryotic cell. I have found that it is common for students in the physical sciences never to have encountered the cytoskeleton before, despite its critical importance to cell function therefore, we will spend a little time introducing the topic. 

The cytoskeleton is a network of different biological filaments that crisscross the cell cytoplasm. These filaments form part of different cellular structures as they associate with each other and other related proteins. Different elements of the cytoskeleton have a variety of specialized roles in cell motility, transport, and division—without a cytoskeleton, cells would not be able to function. There are three main types of filament associated with the cytoskeleton filamentous actin (F-actin), microtubules, and intermediate filaments. Each of these filament types performs different functions within the cellular machinery and as a result have very different mechanical properties. 

The cytoskeleton can be considered a cross-linked polymer gel, but the real structure of the filaments that make up the cytoskeleton is much more complicated than our simple synthetic examples. In a living cell, the cytoskeleton is highly dynamic, and filaments continuously disassemble and reassemble in response to different stimuli. There are many different proteins involved with this reorganization and exact mechanisms are far too numerous to discuss here. 

FIGURE 6.11 The molecular structure of G-actin, a globular protein with a molecular weight of 42 kDa. 

FIGURE 6.12 A transmission electron microscopic image of single F-actin filaments. The filaments are negatively stained with uranyl acetate. 

In order to understand the logic of dynamic instability, we need to keep in mind that cytoskeletal filaments are unstable only when their ends are not attached to particular molecules that have the ability to anchor them. Every microtubule, for example, starts from an organising centre (the centrosome), and the extremity which is attached to this structure is perfectly stable, whereas the other extremity can grow longer or shorter, and becomes stable only when it encounters an anchoring molecule in the cytoplasm. If such an anchor is not found, the whole microtubule is rapidly dismantled and another is launched in another direction, thus allowing the cytoskeleton to explore all the cytoplasm s space in a short time. 

A classic example of this strategy is offered by mitosis. In this case it is imperative that microtubules become attached to the centromeres, so that the chromosomes can be transported to opposite ends of the splindle, but centromes are extremely small and their distribution in space is virtually random. Looking for centromeres is literally like looking for a needle in a haystack, and yet the exploratory mechanism of dynamic instability always finds them, and always manages to find 

It is the anchoring molecules (that strangely enough biologists call accessory proteins) that determine the form that cells have in space and the movements that they perform. The best proof of this enormous versatility is the fact that the cytoskeleton was invented by unicellular eukaryotes, but later was exploited by metazoa to build completely new structures such as the axons of neurons, the myofibrils of muscles, the mobile mouths of macrophages, the tentacles of killer lymphocytes and countless other specialisations. 

We conclude that dynamic instability is a means of creating an endless stream of cell types with only one common structure and with the choice of a few anchoring molecules. But this is possible only because there is no necessary relationship between the common structure of the cytoskeleton and the cellular structures that the cytoskeleton is working on. The anchoring molecules (or accessory proteins) are true adaptors that perform two independent recognition processes microtubules on one side and different cellular structures on the other side. The resulting correspondence is based therefore on arbitrary rules, on true natural conventions that we can refer to as the cytoskeleton codes. 

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The modem era of biochemistry and molecular biology has been shaped not least by the isolation and characterization of individual molecules. Recently, however, more and more polyfunctional macromolecular complexes are being discovered, including nonrandomly codistributed membrane-bound proteins [41], These are made up of several individual proteins, which can assemble spontaneously, possibly in the presence of a lipid membrane or an element of the cytoskeleton [42] which are themselves supramolecular complexes. Some of these complexes, e.g. snail haemocyanin [4o], are merely assembled from a very large number of identical subunits vimses are much larger and more elaborate and we are still some way from understanding the processes controlling the assembly of the wonderfully intricate and beautiful stmctures responsible for the iridescent colours of butterflies and moths [44]. 

Fig. 1. The GP Ib-IX-V complex. The complex consists of seven transmembrane polypeptides denoted GP Iba (mol wt 145,000), GP IbP (mol wt 24,000), GPIX (mol wt 17,000) and GP V (mol wt 82,000), in a stoichiometry of 2 2 2 1. The hatched region represents the plasma membrane. The area above the hatched region represents the extracellular space that below represents the cytoplasm. The complex is a major attachment site between the plasma membrane and the cytoskeleton. Two molecules associated with the cytoplasmic domain are depicted a 14-3-3 dimer, which may mediate intracellular signaling, and actin-binding protein, which connects the complex to the cortical cytoskeleton and fixes its position and influences its function.

J. W. Shay, ed.. Cell and Molecular Biology of the Cytoskeleton, Plenum Press, New York, 1986. 

In this lecture we will be concerned by exocytosis of neurotransmitters by chromaffin cells. These cells, located above kidneys, produce the adrenaline burst which induces fast body reactions they are used in neurosciences as standard models for the study of exocytosis by catecholaminergic neurons. Prior to exocytosis, adrenaline is contained at highly concentrated solutions into a polyelectrolyte gel matrix packed into small vesicles present in the cell cytoplasm and brought by the cytoskeleton near the cell outer membrane. Stimulation of the cell by divalent ions induces the fusion of the vesicles membrane with that of the cell and hence the release of the intravesicular content into the outer-cytoplasmic region. 

Just how fast can proteins move in a biological membrane Many membrane proteins can move laterally across a membrane at a rate of a few microns per minute. On the other hand, some integral membrane proteins are much more restricted in their lateral movement, with diffusion rates of about 10 nm/sec or even slower. These latter proteins are often found to be anchored to the cytoskeleton (Chapter 17), a complex latticelike structure that maintains the cell s shape and assists in the controlled movement of various substances through the ceil. 

C. botulinum C3-toxin and related toxins Rho proteins ADP-ribosylation Inhibition of RhoA, B,C Destruction of the cytoskeleton... 

The cytoplasmic domains of protocadherins are unrelated to those of classical cadherins. They do not bind catenins and it is not clear whether they are associated with the cytoskeleton [1]. Some protocadherins interact with the c- src-related kinase Fyn, indicating a role in signal transduction (see below). 

Interactions with the cytoskeleton seem to be responsible for the processing and the targeting of the Na+/fC+-ATPase to the appropriate compartment structures. Protein kinases are considered to play an essential role in modulation of the sodium pump. 

Role of the Cytoskeleton in Cell Division Formation of the Mitotic Spindle, Mitosis, and Cytokinesis Drug Effects on Microtubules Mlcrofllaments Actin Filaments Structure and Composition... 

The cytoskeleton also contains different accessory proteins, which, in accordance with their affinities and functions, are designated as microtubule-associated proteins (MAPs), actin-binding proteins (ABPs), intermediate-filament-associated proteins (IFAPs), and myosin-binding proteins. This chapter is focused on those parts of the cytoskeleton that are composed of microfilaments and microtubules and their associated proteins. The subject of intermediate filaments is dealt with in detail in Volume 2. 

As plant cells grow, they deposit new layers of cellulose external to the plasma membrane by exocytosis. The newest regions, which are laid down successively in three layers next to the plasma membrane, are termed the secondary cell wall. Because the latter varies in its chemical composition and structure at different locations around the cell, Golgi-derived vesicles must be guided by the cytoskeleton... 

The Cytoskeleton—Microtubules and Microfilaments Drug Effects on Microtubules... 

More than 50 proteins have been discovered in the cytosol of nonmuscle cells that bind to actin and affect the assembly and disassembly of actin filaments or the cross-linking of actin filaments with each other, with other filamentous components of the cytoskeleton, or with the plasma membrane. Collectively, these are known as actin-binding proteins (ABPs). Their mechanisms of actions are complex and are subject to regulation by specific binding affinities to actin and other molecules, cooperation or competition with other ABPs, local changes in the concentrations of ions in the cytosol, and physical forces (Way and Weeds, 1990). Classifications of ABPs have been proposed that are based on their site of binding to actin and on their molecular structure and function (Pollard and Cooper, 1986 Herrmann, 1989 Pollard et al., 1994). These include the following ... 

Figure 5. Diagrammatic representation of the cytoskeleton in the apical region of the intestinal epithelial cell (enterocyte). (This diagram is from data previously published for example, see Mooseker, 1985.)...

The Cytoskeleton—Microtubules and Microfilaments Patterns of Arrangement of Actin Filaments in Animal Cells... 

Thus far, microtubules and actin filaments and their associated proteins have been discussed to advantage as independent cytoskeletal components. In actual fact, all of the components of the cytoskeleton (including intermediate filaments) are precisely integrated with one another (Langford, 1995), as well as with various cytoplasmic organelles, the nuclear membrane, the plasma membrane, and the extracellular matrix. In its totality the cytoskeleton subserves many coordinated and regulated functions in the cell ... 

The cytoskeleton undergoes extensive reorganization during mitosis, and is responsible for the equipartition of a diploid set of chromosomes to each daughter cell (McIntosh and Koonce, 1989 Wadsworth, 1993). 

The cytoskeleton is involved in the maintenance of cell shape and cytoplasmic processes (e.g., microvilli). In polarized epithelial cells, distinct cyto-cortical cytoskeletal complexes are associated with the apical and basal-lateral domains of the plasma membrane (Rodriguez-Boulan and Nelson, 1989 Mays et al., 1994). 

The cytoskeleton is involved in the movement and positioning of cytoplasmic organelles (Cole and Lippincott-Schwartz, 1995). 

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