Cytoplasmic dynein is an AAA(+)-type molecular motor whose major components are two identical heavy chains containing six AAA(+) modules in tandem. It moves along a single microtubule in multiple steps which are accompanied with multiple ATP hydrolysis. This processive sliding is crucial for cargo transports in vivo. To examine how cytoplasmic dynein exhibits this processivity, we performed in vitro motility assays of two-headed full-length or truncated single-headed heavy chains. The results indicated that four to five molecules of the single-headed heavy chain were required for continuous microtubule sliding, while approximately one molecule of the two-headed full-length heavy chain was enough for the continuous sliding. The ratio of the stroking time to the total ATPase cycle time, which is a quantitative indicator of the processivity, was similar to 0.2 for the single-headed heavy chain, while it was similar to 0.6 for the full-length molecule. When two single-headed heavy chains were artificially linked by a coiled-coil of myosin, the processivity was restored. These results suggest that the two heads of a single cytoplasmic dynein communicate with each other to take processive steps along a microtubule. (c) 2006 Elsevier Inc. All rights reserved.