Cytoskeleton/cytoskeletal proteins

Movement. Proteins are involved in all cell movements. For example, actin, tubulin, and other proteins comprise the cytoskeleton. Cytoskeletal proteins are active in cell division, endocytosis, exocytosis, and the ameboid movement of white blood cells. 

Methods for visualizing individual neurons and glia in vivo have depended for more than 100 years on histo-chemical reactions with cytoskeletal elements and even now these methods have not been surpassed. Because cytoskeletal structures play a particularly prominent role in the nervous system, cytoskeletal proteins represent a large fraction of total brain protein, comprising perhaps a third or more of the total. In fact, much of our knowledge about cytoskeletal biochemistry is based on studies of proteins purified from brain. The aims of this chapter are twofold first to provide an introduction to the cytoskeletal elements themselves and second to examine their role in neuronal function. Throughout, the emphasis will be on the cytoskeleton as a vital, dynamic component of the nervous system. 

The NR2 subunit of the NMDA receptor binds to PDZ domains of PSD95 with its cytoplasmic C-terminus. PSD95 also binds a-actinin, which again binds to filamentous actin (F-actin), a main cytoskeletal protein in dendritic spines. In this manner PSD95 anchors the NMDA receptor to the cytoskeleton of the dendritic spine. 

All eucaryotic cells contain various proteins in their cytoplasm that interact to form mechanically stabilizing structures. The amounts of these proteins differ with cell type, and the structural elements - collectively referred to as the cytoskeleton -can be very labile. Labile transformations of cytoskeletal networks are involved in such essential biological phenomena as chromosome movement and cell division, intracellular material transport, shape changes relating to tissue development, and amoeboid-like locomotion (1-3). A great deal of work in recent years has led to the biochemical characterization of numerous cytoskeletal proteins(A) and the elucidation of their spatial localization within a cell(2). However, few quantifiable models yet exist that are appropriate for incorporating that information into notions of shape transformation and cell movement(5-8). 

This process is an early morphological change in cells often seen in isolated cells in vitro but also known to occur in vivo. The blebs, which appear before membrane permeability alters, are initially reversible. However, if the toxic insult is sufficiently severe and the cellular changes become irreversible, the blebs may rupture. If this occurs, vital cellular components may be lost and cell death follows. The occurrence of blebs may be due to damage to the cytoskeleton, which is attached to the plasma membrane as described above. The cause may be an increase in cytosolic Ca2+, interaction with cytoskeletal proteins, or modification of thiol groups (see below). 

Hydroxyprimaquine (Fig. 7.46) will produce ROS in red cells, causing oxidative injury to the cytoskeleton and the oxidation of hemoglobin. The oxidized hemoglobin will bind via disulfide bridges to cytoskeletal proteins (seen as Heinz bodies under the microscope). This and other damages to the red cell lead to their removal by the spleen, and therefore anemia develops. 5-Hydroxyprimaquine also causes a depletion of GSH and the formation of GSS-protein conjugates. This metabolite is considerably more potent than 6-methoxy-8-hydroxylaminoquinoline. 

The contacts between two adjoining cell membranes are stabilized by specific cell adhesion molecules (CAMs), which include the Ca+2-dependent cadherins. These molecules appear to lead the way for cell-cell communications and are involved in mechanochemical transduction via cell-cell interactions. In some cell types, cadherins are concentrated within adherens junctions that are stretch-sensitive and their extracellular domains interact with cadherins on adjacent cells whereas their cytoplasmic domains provide attachment to the actin cytoskeleton via catenins and other cytoskeletal proteins. The Rho family is required for the establishment and maintenance of cadherin-based adherens junctions. The type of cadherin expressed in a cell can affect the specificity and the physiological properties of cell-cell interactions. 

The proteins and other molecules that bind to the inositol lipids will affect their turnover rate and the extent of their impact on cellular metabolism. If so many cytoskeletal proteins bind to the inositol lipids, does this mean that the lipids are associated with the cytoskeleton Ptdlns 3-kinase activity is cytoskeletal associated (Dove et al., 1994) and at least one form of Ptdlns 4-kinase, (the type III Ptdlns 4-kinase) is cytoskeletal associated (Stevenson etal., 1998). 

Vinculin is a cytoskeletal protein, associated with the cytoplasmic side of focal adhesion plaques. Vinculin associates with talin and binds integrins to the cytoskeleton. 

Changes in intracellular calcium homeostasis produced by active metabolites of xenobiotics may cause disruption of the dynamic cytoskeleton. There are a few toxins that cause disruption of the cytoskeleton through mechanisms independent of biotransformation. Microcystin is one of these toxins. Microcystin is produced by the cyanobacterium Microcystis aeruginosa. Similar toxins are produced by other species of cyanobacteria. The hepatocyte is the specific target of microcystin, which enters the cell through a bile-acid transporter. Microcystin covalently binds to serine/threonine protein phosphatase, leading to the hyperphosphorylation of cytoskeletal proteins and deformation of the cytoskeleton (Treinen-Moslen, 2001). 

Inside the muscle fiber there is also a cytoskeleton — the protein structures assuring the integrity of muscle cells. Cytoskeletal proteins such as titin and nebulin are located in myofibrils and anchored in the Z line. Desmin is made up of costamers, which connect the myofibrils vinculin connects myofibrils and sarcolemma. Postmortem changes in cytoskeletal proteins probably play a role in the improvement of meat functional properties, especially its tenderness and water-holding capacity. 

Calpains are preferentially involved in the degradation of proteins of the cytoskeleton. Although other proteolytic systems are also important for the removal of cytoskeletal proteins, the bulk of intracellular proteins are degraded by the proteasomal system [18]. This system is therefore the major proteolytic system involved in the degradation of proteins and in particular of oxidized proteins. The proteasomal system is often referred to as the ubiquitin-proteasome system due to its tight interaction with the ubiquitination machinery of the cell. 

Calpains are cytosolic Ca- -dependent cysteine proteases [45]. They are located close to the cytoskeleton and mostly degrade cytoskeletal proteins, protein kinases, phosphatases, membrane receptors, transport proteins, and regulatory proteins [46,47]. Stimulation of calpain activity was shown following oxidative stress [48-55] in combination with an increase in intracellular Ca concentration [50, 54]. No studies exist that point towards a selective recognition of oxidized protein substrates by calpains [56]. However, it has been demonstrated that the oxidized cytoskeletal protein ezrin is degraded by the proteasome [57]. 

Several different gene families encode the various proteins that make up the cytoskeleton. These proteins are present in varying amounts In almost all cells. In vertebrates, the major cytoskeletal proteins are the actins, tubulins, and intermediate filament proteins like the keratins. We examined the origin of one such family, the tubulin... 

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