Distribution and function of myosin II as a main constituent of the microfilament system in Physarum polycephalum

Abstract

Summary Morphological and physiological investigations on different plasmodial stages of Physarum polycephalum demonstrate a manifold functional significance of the cytoskeletal protein myosin II. During morphogenesis of the complex microfilament system myosin is first incorporated into the submembraneous cell cortex and then into nascent cytoplasmic fibrils with a gradual duplication after 6–8 min. Hence, myosin exhibits a varying topographical distribution in that fibrils of the plasmodial front contain 35% less myosin than fibrils of the uroid region; finally, mature fibrils show a sarcomeric-like arrangement of the motor protein with a periodicity of 0.7 μm. The contraction behavior of comparable specimens after detergent extraction correlates with the myosin content and distribution. Young stages attain only 60% of the contractile efficiency as measured for older ones, and uroid regions contract with a 4 to 6 times higher efficiency than front regions. However, a fibrillar system is not implicitly essential for motive force generation since plasmodial stages that only contain a filament cortex also perform distinct contractions. Altogether, results of the present paper point to the outstanding significance of myosin II in Physarum as a motor producing cytoplasmic streaming, an element stabilizing cytoskeletal organization and a factor probably controlling plasmodial polarity and aging.