Figure 244

Thin filament organization and structure in cardiac cells, a. Immunofluorescence micrograph of a chick cardiac myocyte stained for actin (green) to show the thin filaments and for tropomodulin (red) to show the location of the slow-growing (-) ends of the thin filaments. Tropomodulin appears as regular striations because of the uniform lengths and alignments of the thin filaments in sarcomeres, b. Diagram of a thin filament. The polarity of the thin filament is indicated called filopodia, located around their surface. As in lamel-lipodia, these protrusions contain loose aggregations of 10 to 20 actin filaments organized in the same direction, again with their plus ends directed toward the plasma membrane. Actin filaments are also essential in cytoplasmic streaming, i.e., the stream-like movement of cytoplasm that can be observed in cultured cells.

intermediate Filaments

Intermediate filaments play a supporting or general structural role. These rope-like filaments are called "intermediate" because their diameter of 8 to 10 nm is "intermediate" between that of actin filaments and microtubules. Nearly all intermediate filaments consist of subunits with a molecular weight of about 50 kDa. Some evidence suggests that many of the stable structural proteins in intermediate filaments evolved from highly conserved enzymes, with only minor genetic modification.

Intermediate filaments are formed from nonpolar and highly variable intermediate filament subunits

Unlike those of microfilaments and microtubules, the protein subunits of intermediate filaments show considerable diversity and tissue specificity. In addition, they do not posses enzymatic activity and form nonpolar filaments. Intermediate filaments also do not typically disappear and re-form in the continuous manner characteristic of most microtubules and actin filaments. For these reasons, intermediate filaments are believed to play a primarily structural role within the cell and to compose the cytoplasmic link of a tissue-wide continuum of cytoplasmic, nuclear, and extracellular filaments (Fig. 2.45).

Intermediate filament proteins are characterized by a highly variable central rod-shaped domain with strictly conserved globular domains at either end (Fig. 2.46). Although the various classes of intermediate filaments differ in the amino acid sequence of the rod-shaped domain and show some variation in molecular weight, they all share a homologous region that is important in filament self-assembly. Intermediate filaments are assembled from a pair of helical monomers that twist around each other to form coiled-coil dimers. Then, two coiled-coil dimers twist around each other in antiparallel fashion (parallel but pointing in opposite directions) to generate a staggered tetramer of two coiled-coil dimers, thus forming the nonpolarized unit of the intermediate filaments (see Fig. 2.46). Each tetramer, acting as an individual unit, is aligned along the axis of the filament. The ends of the tetramers are bound together to form the free ends of the filament. This assembly process provides a stable, staggered, helical array in which filaments are packed together and additionally stabilized by lateral binding interactions between adjacent tetramers.

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