CENTROSOME POSITIONING AND DIRECTIONALITY OF CELL MOVEMENTS

In several cell types, an intriguing correlation exists between the po sition of the centrosome and the direction of cell movement: the centr osome is located behind the leading edge, suggesting that it serves as a steering device for directional movement. A logical extension of th is suggestion is that a change in the direction of cell movement is pr eceded by a reorientation, or shift, of the centrosome in the intended direction of movement. We have used a fusion protein of green fluores cent protein (GFP) and gamma-tubulin to label the centrosome in migrat ing amoebae of Dictgostelium discoideum, allowing us to determine the relationship of centrosome positioning and the direction of cell movem ent with high spatial and temporal resolution in living cells. We find that the extension of a new pseudopod in a migrating cell precedes ce ntrosome repositioning. An average of 12 sec elapses between the initi ation of pseudopod extension and reorientation of the centrosome. If n o reorientation occurs within approximately 30 sec, the pseudopod is r etracted. Thus the centrosome does not direct a cell's migration. Howe ver, its repositioning stabilizes a chosen direction of movement, most probably by means of the microtubule system.