Cytoplasmic filaments

Ris, H. (1985). The cytoplasmic filament system in critical point-dried whole mounts and plastic-embedded section. J. Cell Biol. 100,1474-1487. 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

Electron microscopy reveals several types of protein filaments crisscrossing the eukaryotic cell, forming an interlocking three-dimensional meshwork, the cytoskeleton. There are three general types of cytoplasmic filaments— actin filaments, microtubules, and intermediate filaments (Fig. 1-9)—differing in width (from about 6 to 22 nm), composition, and specific function. All types provide structure and organization to the cytoplasm and shape to the cell. Actin filaments and microtubules also help to produce the motion of organelles or of the whole cell. 

Gap junctions (nexus) are broad areas of closely opposed plasma membranes, but there is no fusion of the plasma membranes and a narrow gap, of about 2 to 3 nm wide, remains. The gap is crossed by cytoplasmic filaments, which allow intracellular cytoplasm to transfer between cells. This type of cell junction not only functions as an adherent zone, but also permits the passage of ions and other small molecules (sugars, amino acids, nucleotides and vitamins). Thus the gap junctions are sites of intercellular information exchange. 

Another general feature of the protein folds that confers an important element of fitness is that they generally contain a relatively compact hydrophobic core. This provides a convenient reaction chamber for organic syntheses that are, for the most part, difficult to carry out unless water is excluded. The dense hydrophobic core may also confer on many folds the ability to stack together into various stable supramolecular assemblies that form the basic elements of the cytoskeleton nucleosomes, cytoplasmic filaments, microtubules, and so forth. 

F-actin is observed in three important configurations as filaments, associated with filasomes, and positioned within the nuclear matrix as an inclusion (Fig. ID, right). Treatment of the germlings with cytochalasin E leads to the disappearance of cytoplasmic filaments but has no perceivable effects on the filasomes (27). 

Vimentin is ubiquitous in soft tissue tumors, and is present in poorly differentiated sarcomas as the most primitive of cytoplasmic filaments. 

The Importln-Ran-GTP complex then diffuses back through the NPC, again, through transient interactions of the Importln with FG repeats. Once the Importin-Ran-GTP complex reaches the cytoplasmic side of the NPC, Ran interacts with a specific GTPase accelerating protein (Ran-GAP) that is a component of the NPC cytoplasmic filaments. This stimulates Ran to hydrolyze its bound GTP to GDP, causing It to convert to a conformation that has low affinity for the Importin, so that the free Importin is released Into the cytoplasm, where it can participate in another cycle of Import. 

Another participant in mRNP transport to the cytoplasm Is the nuclear cap-bIndIng complex, mentioned earlier as protection against exonuclease attack on the 5 end of nascent transcripts and pre-mRNAs. Electron microscopy experiments discussed below have demonstrated that the 5 end of mRNAs lead the way through the nuclear pore complex. Recent experiments in yeast indicate that the 3 poly(A) tail plays an Important role In mRNP transport, suggesting that a poly(A)-binding protein participates. Nucleoporins associated with the NPC cytoplasmic filaments In addition to FG-nucleoporins are required for mRNA export and may function to dissociate the mRNA-exporter and other mRNP proteins that accompany the mRNP through the pore. 

The smallest free-living organisms are the Mollicutes [65]. Some glide on surfaces, others swim, but all appear to generate the forces for propulsion by cyclically altering elastic properties of cytoplasmic filaments [66-69]. 

The recent availability of antibodies to cytoskeletal components offers an additional means to study reovirus interactions with cytoplasmic filaments. Although electron-microscopic techniques can elu-dicate filament structure and distribution, the full extent of cytoplasmic filament organization was not known until indirect immunofluorescence techniques were developed (Weber et al., 1975 Brinkley et al., 1978 Lazarides, 1975). Immunofluorescence microscopic techniques indicate that cytoplasmic filaments form networks within the cell cytoplasm. 

Within the epithelium resides a population of bone marrow-derived phagocytic cells (airway dendritic cells orLangerhans cells) that constitute the principal population of antigen-presenting cells in the airways (130,131). They extend long cytoplasmic filaments between the epithelial cells to form a continuous network... 

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