Motility systems actin tubulin

Comparison of Actin-Based and Tubulin-Based Motility Systems... 

A class of motile systems completely different from and unrelated to the actin-myosin contractile systems is used in cellular structures as diverse as the mitotic spindle, protozoan and sperm flagella, and nerve axons. These systems are constructed from microtubules, very long, tubular structures built from a helical wrapping of the protein tubulin (Figure 8.19). There are two kinds of tubulin subunits, oi and each of molecular weight 55,000. They are present in equimolar quantities in the microtubule, which can be considered a helical array of ot-/i dimers. Alternatively, we can view the microtubule as consisting of 13 rows, or protofilaments, of alternating ot and subunits. Because the oi and b units are asymmetrical proteins, with a defined and reproducible orientation in the fiber, the microtubule has a definite sense of direction. 

Certain proteins endow cells with unique capabilities for movement. Cell division, muscle contraction, and cell motility represent some of the ways in which cells execute motion. The contractile and motile proteins underlying these motions share a common property they are filamentous or polymerize to form filaments. Examples include actin and myosin, the filamentous proteins forming the contractile systems of cells, and tubulin, the major component of microtubules (the filaments involved in the mitotic spindle of cell division as well as in flagella and cilia). Another class of proteins involved in movement includes dynein and kinesin, so-called motor proteins that drive the movement of vesicles, granules, and organelles along microtubules serving as established cytoskeletal tracks. ... 

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