Myosin superfamily motors play crucial roles in cellular functions such as motility, cell division and organelle trafficking. Different myosin classes are specialized for motion toward either the (+) or (-) end of actin filaments. Previous work has shown that directionality of recombinant myosins may be altered via the genetic insertion [1] or removal [2,3,4] of structural motifs that redirect the lever arm. We have challenged our understanding of myosin structure and function by constructing novel myosin motors that can reversibly switch their direction of motion in response to an external signal. Our general strategy relies on controlling the effective length of lever arms by triggering helix-coil transitions. In one successful design using [Ca++] as the control signal, we have built myosin VI variants with chimeric lever arms composed of an alpha-actinin fragment [5] fused to two or more calmodulin-binding IQ repeats. In vitro motility assays show that the engineered motors reverse directionality in response to physiological levels of [Ca++], as expected.[1] Tsiavaliaris, G., Fujita-Becker, S. and Manstein, D. J. (2004). Nature, 427, 558-561.[2] Bryant, Z., Altman, D. and Spudich, J. A. (2007). Prot. Natl Acad. Sci. USA,104, 772-777.[3] Park, H., Li, A., Chen, L., Houdusse, A., Selvin, P. R. and Sweeney, H. L. (2007). Proc. Natl Acad. Sci. USA, 104, 778-783.[4] Liao, J., Elting, M. W., Delp, S. L., Spudich, J. A. and Bryant, Z. (2009). J. Mol. Biol, 392, 862-867.[5] Anson, M., Greeves, M. A., Kurzawa, S. E. and Manstein, D. J. (1996). EMBO J.15, 6069-6074.