Fig. 1.

Tug-of-war between kinesin and dynein on endosomes inside Dictyostelium cells. (A) Trajectory of a reversal of an endosome inside a Dictyostelium cell in an x–y plane obtained from video tracking. Prereversal (grey circles) and postreversal motion (black circles) shows a close overlap over long distances, indicating that the reversal is along a single microtubule. (Inset) Part of an agar-flattened Dictyostelium cell is shown. The cell boundary is outlined (black dotted line). Two endosomes (circled) and their trajectories (white lines) are schematized. Note the reversals in direction. (Scale bar, 5 μm.) (B) Motion of another endosome close to a reversal is projected along a microtubule by assuming the microtubule to be a straight line. A slow tug-of-war (TOW) segment is sandwiched between fast motion. The direction of motion (plus or minus) is not determined (see main text). (Upper Inset) A time series of images (150 msec apart) of an endosome during reversal. The microtubule orientation is approximately vertical. Note the slowing down and elongation of the endosome, interpreted as a TOW between motors (also see Movie S1 and Movie S2). (Lower Inset) Elongation of this endosome is quantified manually using images from successive frames of Movie S1. Endosome length (distance between front and rear ends along direction of motion) is plotted as a function of time. There is a 33% increase in the length during TOW. TOW lasts for ≈1.3 sec.