MYOSIN COOPERATES WITH MICROTUBULE MOTORS DURING ORGANELLE TRANSPORT IN MELANOPHORES

Melanophores offer an outstanding system for the study of intracellula r motility. These cells aggregate their pigment-filled melanosomes to the cell center or disperse them throughout the cytoplasm in response to hormonal modulation of intracellular cyclic AMP levels in order to effect color changes in lower vertebrates [1]. Previous work from our laboratory demonstrated a role for microtubule-based motors in melanos ome transport and we succeeded in reconstituting their regulated motil ity along microtubules in vitro [2,3]. Here we demonstrate that, in ad dition to microtubule-mediated motility, melanosomes purified from Xen opus melanophores exhibit unidirectional movement along actin filament s in vitro as well. Immunoblotting analysis shows that these organelle s possess the actin-based organelle motor, myosin-V. In vivo, melanoso mes are able to slowly disperse in the absence of microtubules, and th is slow dispersion requires the integrity of the actin cytoskeleton. F urthermore, in cells with dispersed pigment, disruption of filamentous actin induces a rapid, microtubule-dependent aggregation of melanosom es to the cell center. Our results, together with the accompanying pap er by Rodionov et al. [4], demonstrate that the concerted efforts of b oth microtubule-based and actin-based motors are required for proper m elanosome distribution in melanophores. This is the first example of a biochemically defined organelle in possession of both plus-end and mi nus-end directed microtubule motors and a myosin; coordinated activity of all three motors is essential for organelle motility in vivo. (C) Current Biology Ltd.