Regulated bidirectional motility of melanophore pigment granules along microtubules in vitro

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Abstract
Although many types of membrane-bound organelles rely upon microtubule-based transport for their proper placement within the cytoplasm, the molecular mechanisms that regulate intracellular motility remain largely unknown. To address this problem, we have studied the microtubule-dependent dispersion and aggregation of pigment granules from an immortalized Xenopus melanophore cell line. We have reconstituted pigment granule motility along bovine brain microtubules in vitro using a microscope-based motility assay. Pigment granules, or melanosomes, move along single microtubules bidirectionally; however, analysis of the polarities of this movement shows that melanosomes that have been purified from dispersed cells exhibit mostly plus end-directed motility, while movement of organelles from aggregating cells is biased toward the minus end. Removal of all soluble proteins from the melanosome fractions by density gradient centrifugation does not diminish organelle motility, demonstrating that all the components required for transport have a stable association with the melanosome membranes. Western blotting shows the presence of the plus end-directed motor, kinesin-II, and the minus end-directed motor, cytoplasmic dynein in highly purified melanosomes. Therefore, purified melanosomes retain their ability to move along microtubules as well as their regulated state. Direct biochemical comparison of melanosomes from aggregated and dispersed cells may elucidate the molecular mechanisms that regulate organelle transport in melanophores.