Electron microscope observations on myosin from Physarum polycephalum

• Published in: The Journal of cell biology 1972-03, Vol.52 (3), p.648-663
• Main Author: Nachmias, V.T
• Format: Article
• Language: English
• Subjects: Actins; Actomyosin - analysis; Adenosine triphosphatases; Aggregation; Cell biology; Electron microscopes; Electron microscopy; Enzymes; Microfilaments; Microscopy, Electron; Muscle Proteins - analysis; Myosins - analysis; Myxomycetes - analysis; Osmolar Concentration; plant biochemistry; plant physiology; Rabbits; Solubility; Striated muscle; Viscosity
• ISSN: 0021-9525; 1540-8140
• DOI: 10.1083/jcb.52.3.648

Myosin has been separated from Physarum polycephalum actomyosin in confirmation of the results of Hatano and Tazawa. In an intermediate step, myosin-enriched actomyosin has also been obtained. The mean yield of free myosin was 4.4 mg from 100 g of mold. It was obtained as water-clear solutions at μ = 0.055 with calcium ATPase activity of up to 0.5 μM P i/ min per mg. Negatively stained preparations were examined by electron microscopy. Physarum myosin in 0.5 M KCl interacted with actin from rabbit skeletal muscle to form polarized arrowhead complexes similar to but less regular than those of natural actomyosin from muscle or myosin-enriched Physarum actomyosin. The Physarum myosin-enriched actomyosin at low ionic strength displayed evidence of head-to-tail and tail-to-tail aggregation attributable to the myosin component. Yet Physarum myosin alone did not produce detectable filaments at μ = 0.055 at pH 7, 6.5, or 5.8, nor when dialyzed against 0.01 M ammonium acetate, nor when the dielectric constant of the medium was reduced. However, aggregation approaching the extent of 'thick filaments' up to 0.3 μ long was found in some preparations of myosin-enriched actomyosin put into solutions containing adenosine triphosphate. Myosin alone in such solutions did not form filaments. The results are compatible with the idea that head-to-tail aggregations are favored by actin-myosin interactions in Physarum, possibly due to alignment of the extended or tail portions of this myosin molecule.