The review describes experimental systems at multiple levels of complexity, including single-motor-type in vitro assays, multimotor in vitro assays, purified-organelle in vitro assays, and finally in vivo cellular assays. The simplest level of complexity is a single motor with a cargo or label attached and a microtubule track in an in vitro environment. This has been the predominant type of experiment in the study of molecular motors. Adding in accessory proteins and parts of the transport complex, such as dynactin, is the next level of complexity. From the wide variety of in vivo optical trapping results, the different kinesin-dynein transport systems present quite a bit of complexity. Dynein apparently is dragged behind kinesin during some plus-end directed transport, whereas kinesin routinely releases to allow unhindered minus-end-directed transport. More complex systems, such as systems with more motor types present, or other specialized types of transport promise to add more complex regulatory mechanisms.
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