Functional Coordination of Microtubule and Actin Based Motility in Melanophores. V. I. Rodionov, A. J. Hope, T. M. Svitkina and G.G. Borisy Curr. Biol., 8(3): 165-168, 1998

The fish melanophore has been considered the exemplar system of microtubule-based organelle transport. In this system, a radial array of uniformly polarized microtubules [1] provides a framework on which dynein-related and kinesin-related motors drive pigment granules toward the minus or plus ends, respectively [2-4]. Stimulation of minus-end motors accounts satisfactorily for aggregation of granules at the cell center. Rapid dispersion is clearly microtubule-dependent; however, the uniform distribution of granules throughout the cytoplasm is paradoxical because 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 microtubule-independent motility system in fish melanophores. The system is based on actin filaments and is required for achieving uniform distribution of pigment granules. When it is abrogated, granules accumulate at the cell's margin as predicted for microtubule plus-end motors acting alone. The results presented here demonstrate the functional coordination of microtubule and actin filament systems, a finding which may be of general significance for organelle motility in cytoplasm.

	Figure 1 (45K) - Motility of individual pigment granules in a granule-depleted melanophore fragment with fluorescently labeled microtubules
	Figure 2 (112K) - Microtubule-independent motility of pigment granules
	Figure 3 (362K) - Actin filaments in melanophores
	Figure 4 (106K) - Actin filaments are necessary for achieving uniform distribution of pigment
	Figure 5 (59K) - Model for dispersion of pigment granules in melanophores

	Sequence 1 (452 K) - Corresponds to Figure 1. Motility of individual pigment granules in a granule-depleted melanophore fragment with fluorescently labeled microtubules (see text). A granule, marked by the white dot, moved along an irregular track which did not correspond to a microtubule. Frames were taken at 10 second intervals (whole sequence corresponds to 16 minutes real time).
	Sequence 2 (2 M) - Latrunculin treatment of a cell with dispersed pigment. After treatment of the cell with latrunculin (5 µM) pigment granules redistributed to the cell margin. Frames were taken at 10 second intervals (whole sequence corresponds to 30 minutes real time).
	Sequence 3 (1.3 M) - Corresponds to the left half of Figure 4. Redispersion of pigment granules in a control cell. Redispersion induced by 5 mM caffeine ended with uniform distribution of pigment granules. Frames were taken at 10 second intervals (whole sequence corresponds to 16 minutes real time).
	Sequence 4 (1.1 M) - Corresponds to the right half of Figure 4. Redispersion of pigment granules in lantrunculin-treated cell. After normal aggregation, 5 µM latrunculin was added for 15 min before dispersion was stimulated by caffeine. After addition of caffeine pigment granules accumulated at the cell margin rather than distributed uniformly in the cytoplasm as in control cell. Frames were taken at 10 second intervals (whole sequence corresponds to 11 minutes real time).
	Sequence 5 (531 K) - Pigment aggregation in latrunculin-treated cell. Cell was treated with 5 mM latrunculin for 60 minutes and pigment aggregation was induced by adrenalin. Pigment granules aggregated with normal kinetics. Frames were taken at 10 second intervals (whole sequence corresponds to 12 minutes real time).
	Sequence 6 (938 K) - Pigment redispersion in the cell shown in Sequence 5. Frames were taken at 10 second intervals (whole sequence corresponds to 15 minutes real time).