Morphological aspects of thiophosphorylated and dephosphorylated myosin molecules from the plasmodium of Physarum polycephalum.

Electron microscopically, the myosin molecule from the plasmodium of Physarum polycephalum has a long tail of 173 nm, having a flexible region over the range of 80 to 120 nm from the head-tail junction. In 0.6 M ammonium acetate, this region of the dephosphorylated myosin molecules is more flexible than that of the thiophosphorylated ones. In 50 mM ammonium acetate, the dephosphorylated myosin molecules exist in monomeric and oligomeric forms, independently of ATP and Mg2+, whereas the thiophosphorylated myosin molecules form dense aggregates of thick filaments. The tails of the monomeric dephosphorylated myosin molecules bend sharply at the flexible region at angles of more than 120 degrees. In oligomers of the dephosphorylated myosin molecules, the molecules are all associated side-to-side with straight tails and are oriented in the same direction. Based on these results, the regulation mechanism of cell motility of the plasmodium is discussed.