TABLE Summary Characteristics of Three Types of Cytoskeletal Elements 2.4 Shape Double-stranded linear Ropelike fibers Nonbranching long, hollow helical array cylinders Diameter 6–8 8–10 20–25 (nm) Basic Monomer of G-actin Various intermediate filament Dimers of and -tubulin protein (MW 42 kDa) proteins (MW ~50 kDa) (MW 54 kDa) -tubulin subunit found in MTOC is necessary for nucleation of microtubules -, -, -, -tubulins are associated with MTOC and basal bodies Enzymatic ATP hydrolytic activity None GTP hydrolytic activity activity Polarity Yes Nonpolar structures Yes Minus ( ) or pointed Minus ( ) end is nongrowing end is slow-growing end end embedded in MTOC Plus ( ) or barbed end Plus ( ) end is the growing end is faster-growing end Assembly Monomers of G-actin are Two pairs of monomers At the nucleation site, and process added to growing flament form two coiled-coil -tubulin dimers are added Polymerization requires dimers; then two coiled-to -tubulin ring in an presence of K , Mg2 , and coil dimers twist around end-to-end fashion ATP, which is hydrolyzed to each other to generate Each tubulin dimer ADP after each G-actin a staggered tetramer, molecule binds GTP before it molecule is incorporated which aligns along the becomes incorporated into into the flament axis of the flament and the microtubule binds to the free end of Polymerization also requires the elongating structure presence of Mg2 GTP–tubulin complex is polymerized; after incorporation, GTP is hydrolyzed to GDP Source of ATP N/A GTP energy required for assembly Characteristics Thin, fexible flaments Strong, stable structures Exhibit dynamic instability Associated Variety of ABPs with different Intermediate flament– Microtubule-associated proteins functions: fascin bundling; associated proteins: plectins proteins: MAP-1, 2, 3, gelsolin flament severing; bind micro tubules, actin, and 4, MAP-, and TOG CP protein capping; and intermediate flaments; regulate assembly, stabilize, spectrin cross-linking; desmoplakins and plakoglobins and anchor microtubules to myosin I and II motor attach intermediate flaments specifc organelles; motor functions to desmosomes and proteins—dyneins and kinesins— hemidesmosomes required for organelle movement Actin Filaments (Microfilaments) Intermediate Filaments Microtubules chapter 2 Location Core of microvilli Extend across cytoplasm Core of cilia in cell Terminal web connecting desmosomes Emerge from Concentrated beneath and hemidesmosomes MTOC and spread plasma membrane In nucleus just beneath into periphery of Contractile elements inner nuclear membrane cell of muscles Mitotic spindle, Contractile ring Centrosome in dividing cells Major Provide essential components Provide mechanical strength Provide network functions to contractile elements and resistance to shearing (“railroad tracks”) for of muscle cells (sarcomeres) forces movement of organelles within cell Provide movement for cilia and chromosomes during cell division TABLE Summary Characteristics of Three Types of Cytoskeletal Elements, Cont.2.4 Actin Filaments (Microfilaments) Intermediate Filaments Microtubules
FIGURE 2.51 • Structure of the MTOC. This diagram shows the represents the growing end directed toward the plasma membrane (see Fig. 2.51).
Centrioles provide basal bodies for cilia and flagella and align the mitotic spindle during cell division.
Although centrioles were discovered more than a century ago, their precise functions, replication, and assembly are still under intense investigation. The known functions of centrioles can be organized into two categories:  Basal body formation. One of the important functions of the centriole is to provide basal bodies, which are necessary for the assembly of cilia and flagella (Fig. 2.52).