Cytoskeleton 100-118 PDF

The arrangement of microtubules in a flagellum or cilium. (A) Electron micrograph of the flagellum of a green- alga cell (Chlamydomonas microtubules. (B) Diagram of the parts of a flagellum or cilium. The various projections from the microtubules link the microtubules together and occur at regular...

The arrangement of microtubules in a flagellum or cilium. (A) Electron micrograph of the flagellum of a green- alga cell (Chlamydomonas microtubules. (B) Diagram of the parts of a flagellum or cilium. The various projections from the microtubules link the microtubules together and occur at regular intervals along the length of the axoneme. (C) High-resolution electron tomography image of an outer microtubule doublet showing structural details and features inside the microtubules called microtubule inner proteins (MIPs). Axonemal dynein. CryoEM reconstruction of a sea urchin sperm flagellum showing dynein arms connecting the A microtubule of one doublet with the B microtubule of an adjacent doublet at regular intervals. Sperm axonemal dynein is dimeric. The tail of the molecule binds tightly to an A microtubule, while the two globular heads each have a stalk that connects to an ATP-dependent binding site on a B microtubule (see Figure 16 58). When the heads hydrolyze their bound ATP, they move toward the minus end of the B microtubule, thereby producing a sliding force between the adjacent microtubule doublets in a cilium or flagellum The bending of an axoneme. (A) When axonemes are exposed to the proteolytic enzyme trypsin, the flexible protein links holding adjacent microtubule doublets together are broken. In this case, the addition of ATP allows the motor action of the dynein heads to slide one microtubule doublet against the adjacent doublet. (B) In an intact axoneme (such as in a spermatozoon), the flexible protein links prevent the sliding of the doublet. The motor action therefore causes a bending motion, creating waves or beating motions. Primary cilia. (A) Electron micrograph and diagram of the basal body of a mouse neuron primary cilium. The axoneme of the primary cilium (black arrow) is nucleated by the mother centriole at the basal body, which localizes at the plasma membrane near the cell surface. (B) Centrioles function alternately as basal bodies and as the core of centrosomes. Before a cell enters the cell-division cycle, the primary cilium is shed or resorbed. The centrioles recruit pericentriolar material and duplicate during S phase, generating two centrosomes, each of which contains a pair of centrioles. The centrosomes nucleate microtubules and localize to the poles of the mitotic spindle. Upon exit from mitosis, a primary cilium again grows from the mother centriole. An electron micrograph of intermediate filaments A model of intermediate filament construction The monomer shown in (B) pairs with another monomer to form a dimer, in which the conserved central rod domains are aligned in parallel and wound together into a coiled- coil. (C) Two dimers then line up side by side to form an antiparallel tetramer of four polypeptide chains. Dimers and tetramers are the soluble subunits of intermediate filaments. (D) Within each tetramer, the two dimers are offset with respect to one another, thereby allowing it to associate with another tetramer. (E) In the final 10-nm-diameter filament, tetramers are packed together in a rope-like array, which has 16 dimers (32 coiled-coils) in cross section. Half of these dimers are pointing in each direction. An electron micrograph of intermediate filaments is shown on the upper left (Movie 16.16).  basal cell of epidermis basal lamina defective keratin hemidesmosomes filament network (A) (B) (C) 40 um Figure 16-64 Blistering of the skin caused by a mutant keratin gene. A mutant gene encoding a truncated keratin protein (lacking both the N - and C - terminal domains) was expressed in a transgenic mouse. The defective protein assembles with the normal keratins and thereby disrupts the keratin filament network ni the basal cells of the skin. Light micrographs of cross sections of (A) normal and (B) mutant skin show that the blistering results from the rupturing of cells ni the basal layer of the mutant epidermis (short red arrows). (C) Asketch of three cells ni the basal layer of the mutant epidermis, as observed by electron microscopy. As indicated by the red arrow, the cells rupture between the nucleus and the hemidesmosomes (discussed ni Chapter 19), which connect the keratin filaments to the underlying basal lamina. A ( and B, © 1991 P.A. Coulombe et al. Originally published ni J. Cell Bio/. https://doi.org/10.1083/jcb.115.6.1661. With permission from Rockefeller University Press.) Two types of intermediate filaments in cells of the nervous system. (A) Freeze-etch electron microscopy image of neurofilaments in a nerve cell axon, showing the extensive cross-linking through protein cross-bridges an arrangement believed to give this long cell process great tensile strength. The cross-bridges are formed by the long, nonhelical extensions at the C-terminus of the largest neurofilament protein (NF-H). (B) Freeze-etch image of glial filaments in glial cells, showing that these intermediate filaments are smooth and have few cross- bridges. (C) Conventional transmission electron micrograph of a cross section of an axon showing the regular side-to-side spacing of the neurofilaments, which greatly outnumber the microtubules. Plectin cross-linking of diverse cytoskeletal elements. Plectin (green) is seen here making cross- links from intermediate filaments (blue) to microtubules (red). In this electron micrograph, the dots (yellow) are gold particles linked to anti-plectin antibodies. The entire actin filament network was Svitkina et al. Originally published in J. Cell Biol. http://doi.org/10.1083/jcb.135.4.991. With permission from Rockefeller University Press.) GTP-binding proteins called septins serve as an additional filament system in all eukaryotes except terrestrial plants. Septins assemble into nonpolar filaments that form rings and cage-like structures, which act as scaffolds to compartmentalize membranes into distinct domains or to recruit and organize the actin and microtubule cytoskeletons. In primary cilia, a ring of septin filaments assembles at the base of Cell compartmentalization by septins. (A) Septins form filaments in the the cilium and serves as a diffusion neck region between a mother yeast cell and bud. (B) In this barrier at the plasma membrane, photomicrograph of human cultured cells, the DNA is stained blue and restricting the movement of septins are labeled in green. The microtubules of primary cilia are labeled membrane proteins and with an antibody that recognizes a modified (acetylated) form of tubulin establishing a specific composition (red) that is enriched in the axoneme. (C) A magnified image reveals a in the ciliary membrane collar of septin at the base of the cilium.