Subcellular cytoskeleton

An increase in cAMP and activation of protein kinase A are accompanied, in many cases, by a change in the subcellular location of the protein kinase A holoenzyme and the catalytic subunit. In particular, protein kinase A in cells of the nerve system is often found associated with the cytoskeleton. This association is mediated by the RII subunit. Binding of cAMP to the regulatory subunit releases the catalytic subunit that can phosphorylate substrates in the near vicinity. The released catalytic subunit can also be transferred to other compartments of the cell. Parallel to the increase in cAMP, translocation of the catalytic subunit is observed in many cells from the Golgi apparatus to the nucleus via the cytosol, and is accompanied by stimulation of transcription. 

A primary function of the SH3 domains is to form fimctional oligomeric complexes at defined subcellular sites, frequently in cooperation with other modular domains. SH3 domains are foimd in many proteins associated with the cytoskeleton or with the plasma membrane. Examples are the actin binding protein a-spectrin and myosin lb. Furthermore, SH3 interactions are involved in signal transduction in the Ras pathway (see Chapter 9). 

Cytoplasmically localized protein tyrosine phosphatases have a catalytic domain and other structural elements that specify the subcellular localization and association with effector molecules. These structural elements contain sequence signals for nuclear localization, for membrane association and for association with the cytoskeleton (see Fig. 8.16). The presence of SH2 domains suggests that these molecules might interact with signaling pathways involving growth hormones and receptor tyrosine kinases. 

Cytosol. The liquid portion of the cytoplasm, including the macromolecules but not including the larger structures, such as subcellular organelles or cytoskeleton. 

In the absence of a death signal, most of the pro- and anti-apoptotic members are located in separate subcellular compartments. Anti-apoptotic proteins are inserted in intracellular membranes, mainly the mitochondrial membrane, while some proapoptotic members are located in the cytoplasm or cytoskeleton in an inactive form. They are activated and translocated by apoptotic stimuli to their place of action to perform their functions (Gross et al., 1999). 

Most cells express one or more isoforms of PKC, of which the substrates are localized in different cellular compartments. This means that either the PKC must be brought to the same compartment where the substrate is, or the substrate must be brought to the enzyme. It turned out that the PKC is directed to the substrate. The PKCs find their subcellular locations with the help of targeting proteins. There exist two groups of targeting proteins for PKCs. To one group belong components of the cell structure, the cytoskeleton and the... 

In this section, we consider the properties of monomeric and polymeric actin, as well as the various proteins that assemble actin filaments Into large structures. With this basic understanding of the actin cytoskeleton established, we examine in Section 19.2 how a cell can tailor this framework to carry out various tasks requiring motion of the entire cell or subcellular parts. 

Eukaryotes differ from prokaryotes in their subcellular organization. Eukaryotes all possess a nucleus, cytoskeleton and endomembrane system. Some organelles ubiquitous... 

The most widely accepted physiological function of TOH is its role as a scavenger of free radicals. Thus, it prevents oxidant injury to polyunsaturated fatty acids and thiol-rich proteins in cellular membranes and cytoskeleton. It is thought to preserve the structure and functional integrity of subcellular organelles (Chow, 1991). Each TOH molecule can react with two peroxyl radicals (Eqs. 

The structure of the cell, the shape of the cell surface, and the arrangement of subcellular organelles is organized by three major protein components microtubules composed of tubulin, which move and position organelles and vesicles thin filaments composed of actin, which form a cytoskeleton, and intermediate filaments composed of different fibrous proteins. Actin and tubulin, which are involved in cell movement, are dynamic structures composed of continuously associating and dissociating globular subunits. Intermediate filaments, which play a structural role, are composed of stable fibrous proteins that turn over more slowly. 

Figure 9.8 A schematic representation of the subcellular compartmentalisation of a wine yeast cell. The cell envelope, comprising a ceU waU, periplasm and plasma membrane, surrounds and encases the yeast cytoplasm. The structural organisation of the intraceUnlar miheu, containing organelles such as the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria and vacuoles, is maintained by a cytoskeleton. Several of these organelles derive from an extended intramembranous system and are not completely independent of each other. Adapted from Pretorius (2000).

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