Microtubule-dependent motor proteins hydrolyze ATP and move along a microtubule toward either its plus or its minus end. If the motor protein is multimeric, as in these examples, it can cross-link two adjacent microtubules and move them relative to one another. (A) Some minus-end-directed multimeric motor proteins rearrange microtubules to form a focus of minus ends, where the motor proteins accumulate. (B) If microtubules are aligned so that they are antiparallel (that is, their plus ends are facing in opposite directions), a cross-linking motor protein can slide the microtubules past each other, as shown here for a plus-end-directed motor protein that could elongate the spindle. (Adapted from A.A. Hyman and E. Karsenti, Cell 84:406–410, 1996.)