Cytoplasmic filaments and their assembly into bundles in Physarum plasmodium.

Abstract

Filaments were visualized in the endoplasmic drop ofPhysarum polycephalum. The drop was obtained through puncture of the wall of the plasmodial capillary. A glycerinated specimen of the drop of endoplasm could spread over the surface of aqueous solution in a thin film, which has the outstanding merit of observing cytoplasmic filaments by means of negative staining technique. When the cytoplasmic filaments found on the film was treated with HMM, characteristic arrowhead structures were formed demonstrating that they represented actin filaments. On addition of Mg-ATP, three groups of aggregates were observed: 1. bundles of actin filaments, 2. bundles characteristic of myosin aggregates and 3. loose masses in which actin filaments and myosin aggregates came together. In sectioned specimens, filaments dispersed in the endoplasm without forming large bundles were recognized not infrequently together with other cell organelles. Bundles of filaments were developed in the peripheral region of the drop as gelation of cytoplasm was produced there. They were well preserved in glycerinated specimens and reacted with rabbit HMM to make arrowhead complexes. Among these decorated filaments, non-decorated thicker filaments were also observed. The above observation has made it clear that the fibrillar structures in the peripheral ectoplasmic region of the drop were formed by the assembly of the free filaments in the endoplasm.