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, and , each of molecular weight 55,000. They are present in equimolar quantities in the microtubule, which can be considered a helical array of - dimers. Alternatively, we can view the microtubule as consisting of 13 rows, or protofilaments, of alternating and subunits. Because the and b units are asymmetrical proteins, with a defined and reproducible orientation in the fiber, the microtubule has a definite sense of direction.

The assembly of microtubules bears certain similarities to that of actin, but GTP is required rather than ATP. The - dimers bind GTP and then associate to form oligomers. These oligomers form nucleation sites for the growth of microtubules (Figure 8.20). One end, called the plus end, grows more rapidly than the other, minus end. As in actin polymerization, the nucleotide is hydrolyzed but is held in the filament. The final assembly of a functional microtubule usually involves the binding of other proteins to its surface.