FUNCTIONAL COORDINATION OF MICROTUBULE-BASED AND ACTIN-BASED MOTILITYIN MELANOPHORES

The fish melanophore has been considered the exemplar of microtubule-b ased organelle transport. In this system, a radial array of uniformly polarized microtubules [1] provides a framework on which dynein-relate d and kinesin-related motors drive pigment granules toward the minus o r plus ends, respectively [2-4]. Stimulation of minus-end motors accou nts satisfactorily for aggregation of granules at the cell center. Rap id dispersion is clearly microtubule-dependent; however, the uniform d istribution of granules throughout the cytoplasm is paradoxical becaus e stimulation of plus-end motors is predicted to drive the granules to the cell margin. This paradox suggested that the transport system was incompletely understood. Here, we report the discovery of a microtubu le-independent motility system in fish melanophores. The system is bas ed on actin filaments and is required for achieving uniform distributi on of pigment granules. When it is abrogated, granules accumulate at t he cell's margin as predicted for microtubule plus-end motors acting a lone. The results presented here demonstrate the functional coordinati on of microtubule and actin filament systems, a finding that may be of general significance for organelle motility in cytoplasm. (C) Current Biology Ltd.