Identification of a microfilament-enriched, motile domain in amoeboflagellates of Physarum Polycephalum

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Abstract
A distinctive, transient cytoplasmic domain was identified in haploid cells of Physarum polycephalum that are transforming from amoebae into flagellate swimming cells. As revealed by light microscopy, this region, termed the ridge, is a flattened cytoplasmic extension that displays a specific form of motility, characterized by undulations that propagate rapidly down its long axis. The ridge was further examined by scanning electron microscopy of whole cells, and by transmission electron microscopy of serial sections and negative stain preparations of amoeboflagellate cytoskeletons. We identified three distinct components possibly related to ridge motility: a short flagellum closely associated with the ridge surface, an array of microtubules that is part of a complex microtubular apparatus and a network of filaments that forms a well-defined laminar core within the ridge. Labelling studies with heavy meromyosin subfragment I (S-I) indicated that a major portion of the filament network is actin. Peripheral microfilaments appeared to have a uniform orientation in that arrowheads pointed towards the cell body. Other ordered filament arrangements were apparent with a modified extraction protocol using phalloidin. Similar microfilament organization has been reported in motile regions of other systems, such as the lamellipodia of tissue culture cells. However, the ridge can be distinguished from such other motile regions in that it forms independently of contact with a substratum and is only present in a specific region of amoeboflagellates. Moreover, the ridge appears at a specific time during the course of the transformation. The ridge constitutes a new system for studying the structural basis of cell motility. The arrangements of actin- and microtubule-containing ridge components suggest several testable hypotheses concerning their involvement in cytoskeletal or motile functions.