AbstractUsing video‐enhanced differential interference microscopy and digital image processing, we have observed organelle motility inAcanthamoeba castellanii.In amoebae taken from cultures in rapid growth phase, mitochondria and small particles moved over distances of several microns and at an average velocity of ∼2 μ/s. Mitochondrial motility was verified by intensified fluorescence microscopy of cells that were labeled in vivo with the DNA‐binding dye DAPI or the mitochondria‐specific dye Mito Tracker. We further studied the role of microtubules (MTs) in the translocation of cell organelles. Double‐labelling of fixed cells bules with mitochondrial markers (anti‐F1β antibody, Mito Tracker) and cytoskeletal markers (anti‐tubulin antibody, rhodamine‐phalloidin) demonstrate that the mitochondria colocalize with MTs in the subcortical cell area and are excluded from the F‐actin‐rich cell cortex. Colchicine treatment resluted in an almost complete depolymerization of MTs and an inhibition of organelle motility. Moreover, we have directly visualized MTs in vivo in flattened amoebae. Mitochondria and small particles moved along the MTs in a bidirectional mode at an average velocity of ∼1 μm/s. We conclude that the observed movement of mitochondria and small particles inAcanthamoeba castellaniimainly occurs via microtubules and associated motor proteins
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