Cytoskeleton 65-80 PDF

Figure 16 36 Myosin V walks along actin filaments. (A) The lever arm of myosin V is long, allowing it to take a bigger step along an actin filament than can myosin II (see Figure 16 24). (B) Atomic force microscopy images showing myosin V (green) walking along an actin filament. Myosin V functions...

Figure 16 36 Myosin V walks along actin filaments. (A) The lever arm of myosin V is long, allowing it to take a bigger step along an actin filament than can myosin II (see Figure 16 24). (B) Atomic force microscopy images showing myosin V (green) walking along an actin filament. Myosin V functions to carry cargo in cells. (B, adapted from N. Kodera and T. Ando, Biophys. Rev. 6:237 260, 2014. Reproduced with permission of SNCSC.) Figure 16 37 The structure of a microtubule and its subunit. (A) The subunit of each - -tubulin -tubulin monomer is so tightly bound that it can be -tubulin monomer, however, is less tightly bound and has an important role in filament dynamics. GTP is shown in red. (B -heterodimer) and one protofilament are shown schematically. Each protofilament consists of many adjacent subunits with the same orientation. Figure 16 37 The structure of a microtubule and its subunit. (C) (D) A short segment of a microtubule viewed in an electron microscope. (E) Electron micrograph of a cross section of a microtubule showing a ring of 13 protofilaments. (A, PDB code: 1JFF; D, courtesy of Richard Wade; E, courtesy of Richard Linck.) Figure 16 38 The preferential growth of microtubules at the plus end. Microtubules grow faster at one end than at the other. A stable bundle of microtubules obtained from the core of a cilium (called an axoneme) was incubated for a short time with tubulin subunits under polymerizing conditions. Microtubules grew fastest from the plus end of the microtubule bundle, the end on the right in this micrograph. (Courtesy of Gary Borisy.). Salmon and Clare Waterman Courtesy of Wendy C 3 1 2 3 1 2 4 4 time 0 sec 125 sec 307 sec 669 sec 10 um DYNAMIC INSTABILITY Microtubules depolymerize about 100 times faster from an end containing GDP-tubulin than from one containing GTP-tubulin. AGTP cap favors growth, but fi ti is lost, then depolymerization ensues. GTP cap GROWING SHRINKING Individual microtubules can therefore alternate between a period of growth and a period of rapid disassembly, a phenomenon called dynamic instability. Figure 16 40A Dynamic instability due to the structural differences between a growing and a shrinking microtubule end. (A) If the free tubulin concentration in solution is near the critical concentration, a single microtubule end may undergo transitions between a growing state and a shrinking state. A growing microtubule has GTP-containing subunits at its end, forming a GTP cap. If GTP hydrolysis proceeds more rapidly than subunit addition, the cap is lost and the microtubule begins to shrink, an event called a catastrophe. But GTP-containing subunits may still add to the shrinking end, and if enough add to form a new cap, then microtubule growth resumes, an event called a rescue. More recent studies indicate that free tubulin subunits possess a similar bent conformation in both the T form and the D form. Growing microtubules with curved protofilaments at their tips have now been observed both in vitro and in vivo, and a straightening of the T form containing protofilaments may occur subsequent to subunit incorporation as favorable lateral interactions zip them into the microtubule lattice. Regardless of the mechanism of microtubule assembly, the loss of a GTP cap and subsequent catastrophe causes protofilaments containing D-form subunits to spring apart and depolymerize. Figure 16 41C Microtubule nucleation by the g-tubulin ring complex. (C -TuSC spiral - -TuRC), which is likely to nucleate the minus end of a microtubule as shown here. Note the longitudinal discontinuity between two protofilaments, which results - uniform helical packing of the protofilaments. A centriole consists of a cylindrical array of short, modified microtubules arranged into a barrel shape with striking ninefold symmetry. Together with a large number of accessory proteins, the centrioles recruit the pericentriolar material, where microtubule nucleation takes place. The pericentriolar material consists of a dense spherical matrix that is thought to form through a process of biomolecular condensation. The centrosome duplicates before mitosis, forming a pair of centrosomes that each contain a centriole pair. When mitosis begins, the two centrosomes move apart to form the poles of the mitotic spindle.