FIGURE 18-20 Structure and function of selected members of the kinesin superfamily. Kinesin-1, which includes the original kinesin isolated from squid axons, is a (+) end–directed microtubule motor involved in organelle transport. The kinesin-2 family has two different, but closely related, heavy chains and a third cargo-binding subunit; this class also transports organelles in a (+) end–directed manner. Members of the kinesin-5 family have four heavy chains assembled in a bipolar configuration to interact with two antiparallel microtubules and also move toward the (+) ends. Kinesin-13 family members have the motor domain in the middle of their heavy chains and do not have motor activity, but they do destabilize microtubule ends (see also Figure 18-15a). Additional kinesin family members are mentioned in the text. Different kinesins have been given many different names; we use the unified nomenclature described in C. J. Lawrence et al., 2004, J. Cell Biol. 167:19–22. See R. D. Vale, 2003, Cell 112:467.