Summary. Cytoskeleton elements of a Labyrinthula isolate from theFalkland Islands were studied. The most important characteristic of the genus Labyrinthula is a colourless branched plasmatic network of pseudopodia-like tubes with sliding spindle-shaped uninuclear plasma portions (cell bodies). After fluorescent staining tubulin appears to be uniformly and diffusely distributed throughout the whole network and to form a reticulate structure in the cell bodies. The inhibitor colchicine has no influence on the sliding motility of the cell bodies nor on the movement of the network. Actin is frequently found in the network, partly in the form of microfilament bundles, which are longitudinally arranged. Actin is also present in the cortical region of cell bodies, or of cell body groups. It was difficult to distinguish single cell bodies within groups by fluorescence. The inhibitors cytochalasin B and D stop the movement of cell bodies and network. Myosin is present in the cortical region of each cell body, and the central portions of each individual cell body contain accumulations of this protein. We could not observe any fluorescence in the network after myosin staining with the antibodies we used. An actin-myosin complex is probably responsible for the sliding movement of cell bodies in the Labyrinthula network, because actin is found in the pseudopodia-like tubes, and the cortex of the cell bodies is rich in actin and myosin. This actin-myosin complex seems to differ from another actin-myosin complex that has been postulated to be responsible for the locomotion of psendopodia-like tubes. We propose that two actin-myosin complexes exist. One of them is responsible for locomotory phenomena of the network, and the second for cell body sliding in the pseudopodia-like tubes. In each case the myosin is probably anchored in the inner matrix membrane of the pseudopodia-like tubes. A model for actin-myosin interaction in Labyrinthula spp. is presented.
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