AbstractDynactin is a principal regulator of the minus-end directed microtubule motor dynein. The sidearm of dynactin is essential for binding to microtubules and regulation of dynein activity. Although our understanding of the structure of the dynactin backbone (Arp1 rod) has greatly improved recently, structural details of the sidearm part remain elusive. Here, electron microscopy of individual molecules of the dynactin complex revealed that the sidearm was highly flexible and exhibited diverse morphologies. Utilizing mutants for nanogold labeling and deletion analysis, we determined the domain organization of the largest subunit p150 and identified a filamentous structure protruding from the head domain of the sidearm as the coiled-coil 1 (CC1), the dynein-binding domain, in p150. Furthermore, the protrusion formed by CC1 exhibited either a folded or an extended form, suggesting that CC1 works as an extending “arm”. These findings provide clues to understand how dynactin binds to microtubules and regulates dynein.