Identification and characterization of higher plant myosins responsible for cytoplasmic streaming

Cytoplasmic streaming is essential to the intracellular transport of organelles, vesicles and molecules in plant cells. In many cases, the actin-myosin system is involved in generating the motive force for cytoplasmic streaming. Myosin composed of a 170-kDa (170-kDa myosin) and 175-kDa heavy chain (175-kDa myosin) has been biochemically isolated from lily pollen tubes and tobacco cultured cells (BY-2), respectively. Calmodulin (CaM) is associated with both myosins as a light chain. On the basis of the velocity of F-actin in motility assays in vitro using these myosins and of localization studies using antiserum raised against each heavy chain, we suggested that both myosins are molecular motors for generating the motive force for cytoplasmic streaming in higher plant cells. The 170-kDa myosin is expressed not only in somatic cells but also in germinating pollen. In contrast, the 175-kDa myosin is distributed only in somatic cells. In the tip region of growing pollen tubes, it has been demonstrated that a tip-focused Ca2+ gradient is indispensable for growth and tube orientation. Cytoplasmic streaming in this region has been shown to be inactivated by high concentrations of Ca2+. The motile activity in vitro of 170-kDa myosin is suppressed by low (muM) levels of Ca2+ through its CaM light chain, suggesting that this suppression is one of the mechanisms for inactivating cytoplasmic streaming near the tip region of pollen tubes. The motile activity in vitro of 175-kDa myosin is also inhibited by Ca2+ at concentrations higher than 10(-6) M. it has been revealed that the elevation of cytosolic Ca2+ concentrations causes the cessation of cytoplasmic streaming even in somatic cells. Therefore, Ca2+-sensitivity of the motile activity of myosin appears to be a general molecular basis for Ca2+-induced cessation of cytoplasmic streaming.