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The myosin head consists of a globular catalytic domain that binds actin and hydrolyzes ATP and a (Fig. 1 Upper). Depending on the magnitude of the angle change, the swinging motion of the -8-nm-long lever arm could produce a displacement of similar size, in keeping with 4-to 10-nm steps directly measured (6-9).Electron microscopy has successfully observed two different shapes of the head in a way that is consistent with this model (10). Physical measurements have also supported this hypothesis by demonstrating a cyclic change in gyratory radius of the head during ATP hydrolysis cycles (11,12). Recently, 3-D helical reconstruction of actin decorated with myosin heads revealed a swinging motion of the neck region associated with release of ADP from the catalytic domain, resulting in 3-toThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Motor proteins are able to move protein filaments in vitro. However, useful work cannot be extracted from the existing in vitro systems because filament motions are in random directions on two-dimensional surfaces. We succeeded in restricting kinesin-driven movements of microtubules along linear tracks by using micrometer-scaled grooves lithographically fabricated on glass surfaces. We also accomplished the extraction of unidirectional movement from the bidirectional movements along the linear tracks by adding arrowhead patterns on the tracks. These "rectifiers" enabled us to construct microminiturized circulators in which populations of microtubules rotated in one direction, and to actively transport microtubules between two pools connected by arrowheaded tracks in the fields of micrometer scales.

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Organelle movement is essential for efficient cellular function in eukaryotes. Chloroplast photorelocation movement is important for plant survival as well as for efficient photosynthesis. Chloroplast movement generally is actin dependent and mediated by blue light receptor phototropins. In Arabidopsis thaliana, phototropins mediate chloroplast movement by regulating short actin filaments on chloroplasts (cp-actin filaments), and the chloroplast outer envelope protein CHUP1 is necessary for cp-actin filament accumulation. However, other factors involved in cp-actin filament regulation during chloroplast movement remain to be determined. Here, we report that two kinesin-like proteins, KAC1 and KAC2, are essential for chloroplasts to move and anchor to the plasma membrane. A kac1 mutant showed severely impaired chloroplast accumulation and slow avoidance movement. A kac1kac2 double mutant completely lacked chloroplast photorelocation movement and showed detachment of chloroplasts from the plasma membrane. KAC motor domains are similar to those of the kinesin-14 subfamily (such as Ncd and Kar3) but do not have detectable microtubule-binding activity. The C-terminal domain of KAC1 could interact with F-actin in vitro. Instead of regulating microtubules, KAC proteins mediate chloroplast movement via cp-actin filaments. We conclude that plants have evolved a unique mechanism to regulate actin-based organelle movement using kinesin-like proteins.cp-actin | blue light | organelle movement | phototropin