Cross-linking cytoskeleton

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 ... 

Cross-link MFs in membrane cytoskeleton Links MF/spectrin to membrane proteins... 

Transglutaminases. Ca2+-dependent transglutaminase activation was detected in dying and apoptotic cells its actual role can be related to cross-linking of plasmalemmal proteins with cytoskeleton (Orrenius et al., 2003). 

Goode BL, Wong JJ, Butty AC et al. Coronin promotes the rapid assembly and cross-linking of actin filaments and may link the actin and microtubule cytoskeletons in yeast. J Cell Biol 1999 144 83-98. 

A FIGURE 5-31 Cortical cytoskeleton supporting the plasma membrane in human erythrocytes, (a) Electron micrograph of the erythrocyte membrane showing the spoke-and-hub organization of the cytoskeleton. The long spokes are composed mainly of spectrin and can be seen to intersect at the hubs, or membrane-attachment sites. The darker spots along the spokes are ankyrin molecules, which cross-link spectrin to... 

Cross-links between microtubules and neurofilaments are seen in micrographs of nerve-cell axons (see Figure 19-32). Although the identity of these connections in axons is unknown, they may be IFAPs whose function Is to cross-link neurofilaments and microtubules into a stable cytoskeleton. Alternatively, these connections to microtubules may be the long arms of NF-H, which is known to bind microtubules. 

In the preceding Sects. 3.4.1 and 3.4.2 we have described the QCM response to a disintegration of the actin cytoskeleton on the one hand and to cross-linking of all cellular protein by chemical fixatives on the other. The data implied that cell stiffness brought about by the cytoskeleton determines the acoustic response. However, the experimental means to arrive at this conclusion were rather drastic, in particular when chemical fixatives were applied. With respect to future applications of the QCM in cell biology it seems important to understand whether the technique is sensitive enough to monitor physiological alterations in cell stiffness. We therefore tested the QCM approach on an established phenomenon that has been published recently. 

Fig. 44.10. A generalized view of the erythrocyte cytoskeleton. A. The major protein, spectrin, is linked to the plasma membrane either through interactions with ankyrin and band 3, or with actin, band 4.1, and glycophorin. Other proteins in this complex, but not shown, are tropomyosin and adducin. B. A view from inside the cell, looking up at the cytoskeleton. This view displays the cross-linking of the sprectrin dimers to actin and band 3 anchor sites.

The fungal hyphae grow at the tip (apex). Cell wall material is transported in vesicles from older parts towards the tip region as described earlier. The newly formed cell wall at the tip is relatively plastic. The tip is stabilised by the actin cytoskeleton. Further back from the tip the cell wall becomes more cross-linked and thus stronger (Fig 3.1)... 

The actin cytoskeleton is complex, with over 100 different actin-inter-acting proteins. This section will focus on aspects of the actin cytoskeleton that are specifically related to cell motility. The critical features of the actin cytoskeleton that contribute to cell motility are polymerization/ depolymerization of actin filaments, cross-linking of filaments, and tension generation (through myosin-induced contractile activity) [229]. 

Ibarrondo, F.J., Torres, M. and Coates, T.D. (1999). Periodic formation of nascent lamellae is driven by changes in the stable F-actin pool of polymorphonuclear neutrophils after stimulation with chemotactic peptide and cross-linking of GDIS or GD61. Cell Motil. Cytoskeleton 44, 234—247. 

Because the cross-linked network of the cytoskeleton is the stmcture that provides elastic chararter to an otherwise liquid cytoplasm and acts as the major substrate for motor proteins, much effort and attention has focused on the q oskeleton s role in cell mechanics and motility. The fundamental link between cytoskeletal filaments and motor proteins that enables intraceUulat transport suggests that the cytoskeleton might have emerged as a mechanism for cells to become larger than the roughly 1 pm size limit of bacteria where thermal diffusion of solutes suffices for their cytoplasmic distribution. At the... 

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