Microtubule aster formation by dynein-dependent organelle transport.

The interplay between microtubules and the motor enzyme, cytoplasmic dynein, is essential for organisation of the cytoplasm, organelle transport, and cell division in eukaryotic cells. During mitosis, cytoplasmic dynein organises microtubules into two spindle pole asters, as well as the comparable multiple cytoplasmic asters induced by the microtubule-stabilising agent taxol. The mechanisms behind this cell cycle-regulated organisation are, however, not fully understood. We report here that the unidirectional dynein-dependent pigment organelle aggregation in taxol-treated melanophores from Atlantic cod, induces multiple microtubule asters. Usually, the pigment aggregates to a central pigment mass in the cell center, but pigment aggregation in taxol-treated cells induces formation of several peripheral pigment clusters that each localise to the center of a microtubule aster formation. When a cell with previously formed peripheral pigment clusters redisperse pigment, the asters disappear. Upon a subsequent reaggregation of the pigment, the aster formations reappear. The results indicate that the pigment aggregation process organises the microtubules into these formations. Immuno-electron microscopy of isolated pigment organelles indicates the presence of several dynein molecules on each pigment organelle, making it possible for each organelle to interact with several microtubules and thereby focusing microtubule minus ends. The possibility of unidirectional dynein-dependent organelle movement for organising microtubules into asters during cell division, and similarities in signal transduction between mitosis and pigment movement, are discussed.