MICROTUBULE-ASSOCIATED PROTEIN-DEPENDENT BINDING OF PHAGOSOMES TO MICROTUBULES

In macrophages, phagosome movement is microtubule-dependent. Microtubu les are a prerequisite for phagosome maturation because they facilitat e interact-ions between phagosomes and organelles of the endocytic pat hway. We have established an in vitro assay that measures the binding of purified phagosomes to microtubules. This binding depends on the pr esence of membrane proteins, most likely integral to the surface of ph agosomes, and on macrophage cytosol. The cytosolic binding factor can interact with microtubules prior to the addition of phagosomes to the assay, suggesting that it is a microtubule-associated protein (MAP). C onsistent with this, depletion of MAPs from the cytosol by microtubule affinity removes all binding activity. Microtubule motor proteins sho w no binding activity, whereas a crude MAP preparation is sufficient t o support binding and to restore full binding activity to MAP-depleted cytosol. We show that the activating MAP factor is a heat-sensitive p rotein(s) that migrates at around 150 kDa by gel filtration.