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J. Biol. Chem., Vol. 275, Issue 28, 21618-21623, July 14, 2000

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Truncation of a Mammalian Myosin I Results in Loss of Ca²⁺-sensitive Motility^*

Cynthia Perreault-Micale, Alexander D. Shushan, and Lynne M. Coluccio^§

From the Boston Biomedical Research Institute, Watertown, Massachusetts 02472

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 MI^1IQ. Association of calmodulin to MI^1IQ is Ca²⁺-insensitive. MI^1IQ translocates actin filaments in vitro at a rate resembling MI¹³⁰, but unlike MI¹³⁰, does not exhibit sensitivity to 0.1-100 µM 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.

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