IF Networks Form Various Supportive Structures and Are Connected to Cellular Membranes

A network of intermediate filaments is often found as a laminating layer adjacent to a cellular membrane, where it provides mechanical support. The best example is the nuclear lamina along the inner surface of the nuclear membrane (see Figure 21-16). This supporting network is composed of lamin A and lamin C filaments cross-linked into an orthogonal lattice, which is attached by lamin B to the inner nuclear membrane through interactions with a lamin B receptor, an IFAP, in the membrane. Like the membrane skeleton of the plasma membrane, the lamin nuclear skeleton not only supports the inner nuclear membrane but also provides sites where nuclear pores and interphase chromosomes attach. Thus, the nuclear lamins organize the nuclear contents from the outside in.

In addition to forming the nuclear lamina, intermediate filaments are typically organized in the cytosol as an extended system that stretches from the nuclear envelope to the plasma membrane (see Figure 19-31). Some intermediate filaments run parallel to the cell surface, whereas others traverse the cytosol; together they form an internal framework that helps support the shape and resilience of the cell. The results of in vitro binding experiments suggest that, at the plasma membrane, vimentin filaments bind two proteins: ankyrin, the actin-binding protein associated with the Na+/K+ ATPase in nonerythroid cells, and plectin, which also binds to a6^4 integrin in certain cell junctions (Chapter 6). Through these two IFAPs, the vimentin cytoskeleton is attached to the plasma membrane, providing a flexible structural support.

In muscle, a lattice composed of a band of desmin filaments surrounds the sarcomere (Figure 19-36). The desmin filaments encircle the Z disk and are cross-linked to the plasma membrane by several IFAPs, including paranemin and ankyrin. Longitudinal desmin filaments cross to neighboring Z disks within the myofibril, and connections between desmin filaments around Z disks in adjacent myofibrils serve to cross-link myofibrils into bundles within a muscle cell. The lattice is also attached to the sarcomere through interactions with myosin thick filaments. Because the desmin filaments lie outside the sarcomere, they do not actively participate in generating contractile forces. Rather, desmin plays an essential structural role in maintaining muscle integrity. In transgenic mice lacking desmin, for example, this supporting architecture is disrupted and muscles are misaligned.

▲ FIGURE 19-36 Diagram of desmin filaments in muscle.

These type III Intermediate filaments encircle the Z disk and make additional connections to neighboring Z disks. The alignment of desmin filaments with the muscle sarcomere is held in place at the Z disk by a collar of desmin/synemin heteropolymers.

In Chapter 6, we describe the linkage between keratin filaments in epithelial cells and two types of anchoring junctions: desmosomes, which mediate cell-cell adhesion, and hemidesmosomes, which are responsible for attaching cells to the underlying extracellular matrix. In the electron microscope, both junctions appear as darkly staining pro-teinaceous plaques that are bound to the cytosolic face of the plasma membrane and attached to bundles of keratin filaments (see Figure 6-8). The keratin filaments in one cell are thus indirectly connected to those in a neighboring cell by desmosomes or to the extracellular matrix by hemidesmo-somes. As a result of these connections, shearing forces are distributed from one region of a cell layer to the entire sheet of epithelial cells, providing strength and rigidity to the entire epithelium. Without the supporting network of intermediate filaments, an epithelium remains intact, but the cells are easily damaged by abrasive forces. Like actin microfilaments, which are attached to a third type of cell junction in epithelial cells, intermediate filaments form a flexible but resilient framework that gives structural support to an epithelium.

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