Figure 1.

Schematic representation of (a) porters and (b) sliders. In panel a, a vesicular cargo (represented by the gray ellipse) is being hauled by a two-headed kinesin (with the two heads denoted by black circles) walking on an MT protofilament that consists of periodic repetition of $\alpha -\beta$ tubulin dimers. In b, the sliding of the two MTs, which are antiparallel to each other, takes place when the heads of a four-headed kinesin (say, Eg5) step toward the + ends of the respective MTs, as indicated by the curved arrows. The directions of the resulting movement of the MTs are indicated by the straight arrows.