When ATP is added to membrane-cytoskeletons prepared from Dictyostelium amoebae by the method described previously (S. Yumura and T. Kitanishi-Yumura, Cell Struct. Funct. 15, 355–364, 1990), myosin II is released from the membrane-cytoskeletons after contraction. Simultaneously, the heavy chains of myosin II are phosphorylated by a putative myosin II heavy-chain kinase, at foci within the actin network, with the resultant disassembly of filaments. In this study, we examined factors that control the release of myosin II from the membrane-cytoskeletons, on the assumption that inhibition of the release of myosin II keeps the myosin II in the cortical region, and is responsible for the localization of myosin II in the cortical region. The release of myosin II is inhibited at pH values below 6.5. This effect is not due to the inhibition of heavy-chain phosphorylation but is due to the suppression of disassembly of the filaments. In the membrane-cytoskeletons of aggregating cells, the release of myosin II is inhibited by Ca2+, and this effect is enhanced by pretreatment with calmodulin. In the membrane-cytoskeletons of vegetative cells, the release of myosin II is inhibited by pretreatment with calmodulin, and this effect is Ca2+-independent. The inhibition of the release of myosin II by Ca2+ and/or calmodulin is due to the inhibition of heavy-chain phosphorylation, and calmodulin is associated with the foci within the actin network. These results represent a possible mechanism for the intracellular localization of myosin II via regulation of the release of myosin from the cortical region by changes in intracellular pH and/or intracellular concentrations of Ca2+.
Keywords: myosin II; Dictyostelium; pH; Ca2+; calmodulin.