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Cover   Cell motility, cytokinesis, karyokinesis, vesicular transport, and changes in cell shape are examples of important cell biological processes that are driven by a myriad of molecular motors. Understanding how molecular motors work therefore is of great significance to many aspects of cell and developmental biology. Such an understanding requires a reliable assay for measuring biochemically the function of interest, movement. The cover illustrates the discovery of the first quantitative in vitro motility assay (M.P. Sheetz and J.A. Spudich, 1983. Nature, 303, 31-35). Myosin-coated plastic beads move along actin filaments bound to rows of chloroplasts that are bound to the cytoplasmic side of the plasma membrane of the alga Nitella. The movement of beads coated with skeletal muscle myosin occurred at velocities near those observed in live muscles shortening under zero load. Because the orientation of the actin tracks reverses about the zone somewhat depleted in chloroplasts, beads in the upper zone moved in one direction, whereas beads in the lower zone moved in the other. Simpler, biochemically pure, assays had been attempted for years, without convincing results. Nitella offered an oriented actin track surface (Y.M. Kersey et al. 1976. Proc. Natl. Acad. Sci. USA 73, 165-167) to replace those actin-coated surfaces tried in earlier experimentspurified actin filaments bound to avidin-coated surfaces by their severin- and biotin-labeled barbed ends. Armed with the Nitella results, which allowed one to be sure the myosin-coated beads were active, the biochemically purified system finally yielded and provided the first proof that all one needs for movement at velocities approaching those observed in live muscle under zero load was pure actin, pure myosin, and ATP (J.A. Spudich, S.J. Kron, and M.P. Sheetz, 1985. Nature 315, 584-586). James A. Spudich

Reprinted by permission from Nature (M.P. Sheetz and J.A. Spuduich. 1982. Nature, 303, 31-35.) Copyright 1983 MacMillan Magazines