Engineering a Molecular Railroad

It has been almost 20 yr since Spudich and his colleagues demonstrated purified fluorescent actin filaments writhing on a surface coated with purified myosin in an inverted in vitro motility assay (1). These assays, and the many techniques that have been developed since to manipulate and modify filament-associated motors in vitro, have led many to believe that biological motors will become important nanoengineering components. Still, only the first small steps have been taken toward packaging these remarkable motors into practical and useful mechanisms. Motor proteins have several engineering limitations, one of the most important being the limited range of chemical and physical conditions under which they are stable and operative. With these limitations in mind, we believe that the first practical implementation of motor proteins may be in microdevices implanted into physiological systems. The motors in such a device may transport system components between compartments as on a conveyor belt, may actuate gates or valves, or may power a microgenerator by scavenging energy from biological hosts in the form of ATP. Critical hurdles to nanoengineering with motor proteins include the development of compatible interfaces with the cold hard materials of conventional micro- and nanoengineering, the development of techniques to lay the filamentous tracks in precisely defined arrays, the development of mechanisms to couple cargos to the motors, and the development of control systems to throttle motor action and to load and unload cargo. In this chapter, we review progress towards addressing these challenges, focusing on the kinesin family of microtubule stepping motors.