DISTRIBUTION AND FUNCTION OF MYOSIN-II AS A MAIN CONSTITUENT OF THE MICROFILAMENT SYSTEM IN PHYSARUM-POLYCEPHALUM

Morphological and physiological investigations on different plasmodial stages of Physarum polycephalum demonstrate a manifold functional sig nificance 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 fibri ls with a gradual duplication after 6-8 min. Hence, myosin exhibits a varying topographical distribution in that fibrils of the plasmodial f ront contain 35% less myosin than fibrils of the uroid region; finally , mature fibrils show a sarcomeric-like arrangement of the motor prote in with a periodicity of 0.7 mu m. The contraction behavior of compara ble specimens after detergent extraction correlates with the myosin co ntent and distribution. Young stages attain only 60% of the contractil e efficiency as measured for older ones, and uroid regions contract wi th a 4 to 6 times higher efficiency than front regions. However, a fib rillar system is not implicitly essential for motive force generation since plasmodial stages that only contain a filament cortex also perfo rm distinct contractions. Altogether, results of the present paper poi nt 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.