A ROLE FOR MICROTUBULE DYNAMICS IN PHAGOSOME MOVEMENT

We have shown previously that intracellular phagosome movement require s microtubules. Here we provide evidence that within cells phagosomes display two different kinds of microtubule-based movements in approxim ately equal proportions, The first type occurs predominantly in the ce ll periphery, often shortly after the phagosome is formed, and at spee ds below 0.1 mu m/second. The second is faster (0.2-1.5 mu m/second) a nd occurs mainly after phagosomes have reached the cell interior. Trea ting cells with nanomolar concentrations of taxol or nocodazole alters microtubule dynamics without affecting either total polymer mass or m icrotubule organisation. Such treatments slow the accumulation of phag osomes in the perinuclear region and reduce the number of slow movemen ts by up to 50% without affecting the frequency of fast movements, Thi s suggests that a proportion of slow movements are mediated by microtu bule dynamics while fast movements are powered by microtubule motors, In macrophages, interphase microtubules radiate from the microtubule o rganising centre with their plus-end towards the cell periphery To und erstand the behaviour of 'early' phagosomes at the cell periphery we i nvestigated their ability to bind microtubule plus-ends in vitro, We s how that early phagosomes have a strong preference for microtubule plu s-ends, whereas 'late' phagosomes do not, and that plus-end affinity r equires the presence of microtubule-associated proteins within cytosol , We suggest that phagosomes can bind to the plus-ends of dynamic micr otubules and move by following their shrinkage or growth.