Truncation of a mammalian myosin I results in loss of Ca2+-sensitive motility

MYR-1, a mammalian class I myosin, consisting of a heavy chain and 4-6 associated calmodulins, is represented by the 130-kDa myosin I (or MI130) from rat liver. MI130 translocates actin filaments in vitro in a Ca2+- regulated manner. A decrease in motility observed at higher Ca2+ concentrations has been attributed to calmodulin dissociation. To investigate mammalian myosin I regulation, we have coexpressed in baculovirus calmodulin and an epitope-tagged 85-kDa fragment representing the amino-terminal catalytic motor domain and the first calmodulin-binding IQ domain of rat myr-1; we refer to this truncated molecule here as MI1IQ. Association of calmodulin to MI1IQ is Ca2+-insensitive. MI1IQ translocates actin filaments in vitro at a rate resembling MI130, but unlike MI130, does not exhibit sensitivity to 0.1-100 mu M Ca2+. In addition to demonstrating successful expression of a functional truncated mammalian myosin I in vitro, these results indicate that: 1) Ca2+-induced calmodulin dissociation from MI130 in the presence of actin is not from the first IQ domain, 2) velocity is not affected by the length of the IQ region, and 3) the Ca2+ sensitivity of actin translocation exhibited by MI130 involves 1 or more of the other 5 IQ domains and/or the carboxyl tail.