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In several cell types, an intriguing correlation exists between the position of the centrosome and the direction of cell movement: the centrosome is located behind the leading edge, suggesting that it serves as a steering device for directional movement. A logical extension of this suggestion is that a change in the direction of cell movement is preceded by a reorientation, or shift, of the centrosome in the intended direction of movement. We have used a fusion protein of green f luorescent protein (GFP) and ␥-tubulin to label the centrosome in migrating amoebae of Dictyostelium discoideum, allowing us to determine the relationship of centrosome positioning and the direction of cell movement with high spatial and temporal resolution in living cells. We find that the extension of a new pseudopod in a migrating cell precedes centrosome repositioning. An average of 12 sec elapses between the initiation of pseudopod extension and reorientation of the centrosome. If no reorientation occurs within approximately 30 sec, the pseudopod is retracted. Thus the centrosome does not direct a cell's migration. However, its repositioning stabilizes a chosen direction of movement, most probably by means of the microtubule system.

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We describe rblA, the Dictyostelium ortholog of the retinoblastoma susceptibility gene Rb. In the growth phase, rblA expression is correlated with several factors that lead to 'preference' for the spore pathway. During multicellular development, expression increases 200-fold in differentiating spores. rblA-null strains differentiate stalk cells and spores normally, but in chimeras with wild type, the mutant shows a strong preference for the stalk pathway. rblA-null cells are hypersensitive to the stalk morphogen DIF, suggesting that rblA normally suppresses the DIF response in cells destined for the spore pathway. rblA overexpression during growth leads to G1 arrest, but as growing Dictyostelium are overwhelmingly in G2 phase, rblA does not seem to be important in the normal cell cycle. rblA-null cells show reduced cell size and a premature growth-development transition; the latter appears anomalous but may reflect selection pressures acting on social ameba.