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biochem 3 teus 14.10 - Biochemistry Wednesday Intermediate...

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Biochemistry Wednesday, October 15, 2008 Intermediate Filaments Support the Nuclear Membrane and Help Connect Cells into Tissues Intermediate filaments typically crisscross the cytosol, forming an internal framework that stretches from the nuclear envelope to the plasma membrane (Figure 5-32) . A network of intermediate filaments is located adjacent to some cellular membranes, where it provides mechanical support. For exampl e, lamin A and lamin C filaments form an orthogonal lattice that is associated with lamin B . The entire supporting structure, called the nuclear lamina, is anchored to the inner nuclear membrane by prenyl anchors on lamin B. At the plasma membrane, intermediate filaments are attached by adapter proteins to specialized cell junctions called desmosomes and hemidesmosomes, which mediate cell–cell adhesion and cell–matrix adhesion, respectively, particularly in epithelial tissues. In this way, intermediate filaments in one cell are indirectly connected to intermediate filaments in a neighboring cell or to the extracellular matrix. Because of the important role of cell junctions in cell adhesion and the stability of tissues, we consider their structure and relation to cytoskeletal filaments in detail in Chapter 6. ------------------------------------------------ FIGURE 5-32 Fluorescence micrograph of a PtK2 fibroblast cell stained to reveal keratin intermediate filaments. A network of filaments crisscrosses the cell from the nucleus to the plasma membrane. At the plasma membrane, the filaments are linked by adapter proteins to two types of anchoring junctions: desmosomes between adjacent cells and hemidesmosomes between the cell and the matrix. ---------------------------------------------------------------------------------------------- 1
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Microtubules Radiate from Centrosomes and Organize Certain Subcellular Structures Like microfilaments and intermediate filaments, microtubules are not randomly distributed in cells. - Rather, microtubules radiate from the centrosome, which is the primary microtubule-organizing center (MTOC) in animal cells (Figure 5-33). - As detailed in Chapter 20, the two ends of a microtubule differ in their dynamic properties and are commonly designated as the (+) and (-) ends. For this reason, microtubles can have two distinct orientations relative to one another and to other cell structures. - In many non dividing animal cells, the MTOC is located at the center of the cell near the nucleus, and the radiating microtubules are all oriented with their (+) ends directed toward the cell periphery. - Although most interphase animal cells contain a single perinuclear MTOC, epithelial cells and plant cells contain hundreds of MTOCs. Both of these cell types exhibit distinct functional or structural properties or both in different regions of the cell. - The functional and structural polarity of these cells is linked to the orientation of microtubules within them.
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