Cytoskeleton, changes

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 ... 

It is of interest that proteins termed motility factors (55-70 kD) are secreted by fetal cells and some tumor cells. These proteins act as autocrine factors and stimulate rapid movement by these cells. Motility factors induce the formation of cell processes that are packed with actin filaments and have an increased number of receptors for the matrix proteins laminin and fibronectin. The latter enhance the ability of the cells to bind to the extracellular matrix. Thus, it is likely that motility factors influence the organization of the cytoskeleton through changes taking place at the cell surface (reviewed by Warn and Dowrick, 1989). 

A three-dimensional meshwork of proteinaceous filaments of various sizes fills the space between the organelles of all eukaryotic cell types. This material is known collectively as the cytoskeleton, but despite the static property implied by this name, the cytoskeleton is plastic and dynamic. Not only must the cytoplasm move and modify its shape when a cell changes its position or shape, but the cytoskeleton itself causes these movements. In addition to motility, the cytoskeleton plays a role in metabolism. Several glycolytic enzymes are known to be associated with actin filaments, possibly to concentrate substrate and enzymes locally. Many mRNA species appear to be bound by filaments, especially in egg cells where they may be immobilized in distinct regions thereby becoming concentrated in defined tissues upon subsequent cell divisions. 

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. 

Glycophorin A is also considered to be involved in the binding of such important biological metal-ions as Ca " and Mg to the red-cell mem-brane. " This phenomenon may be important, because it has now been shown that glycophorin is a crucial component in the cytoplasmic, Ca -induced, morphological changes observed in red blood cells. This may result from the fact that glycophorin interacts with the proteins that create the red-cell, cytoskeleton structure. ... 

To diffuse rapidly in the plane of the membrane (lateral diffusion), a molecule must simply move around in the lipid environment (including the polar head groups). It need not change how it interacts with phospholipids or with water since it is constantly exposed to pretty much the same environment. Lateral diffusion can be slowed (or prevented) by interactions between membrane proteins and the cellular cytoskeleton. This spatially restricts a plasma membrane protein to a localized environment. 

Myelin affects axonal structure. The presence of a myelin sheath affects the structure of the axon that it surrounds [5], presumably optimizing its properties for transmission of action potentials by saltatory conduction. Generally, one of the effects of myelin is to increase axonal diameter by inducing biochemical changes in components of the axonal cytoskeleton such as neurofilaments (see Ch. 8). The effects of myelin on axonal structure imply... 

Di Campli, A., Valderrama, F., Babia, T., De Matteis, M. A., Luini, A. and Egea, G. Morphological changes in the Golgi complex correlate with actin cytoskeleton rearrangements. Cell Motil. Cytoskeleton 43 334-348,1999. 

Two general mechanisms have been considered by which a depleted intracellular Ca2+ pool might communicate with the plasma membrane [4]. There is evidence that the IP3 receptor is associated with the cytoskeleton, and this association may tether the IP3 receptor to the plasma membrane. Depletion of intracellular calcium stores might cause a conformational change in the IP3 receptor, which could be conveyed to the plasma membrane via the cytoskeleton or by a more direct protein-protein interaction. Alternatively, signaling could occur... 

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