One can label beads with kinesin or dyneins and watch the direction of movement in a cell at the light microscopic level. What would happen if the beads were simply labeled with "cytoplasmic extract"? This cartoon shows the motility process in vitro. The tubule is moving along a negatively charged glass surface and the vesicle moves along the tubule.
This electron micrograph shows microtubules in cross section with the MAP bridge. The arrows point to bridges between microtubules. The star points to a MAP bridge to the vesicle. In summary, MAPs accelerate polymerization, serve as "motors" for vesicles and granules, and essentially control cell compartmentation.
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