THE ACTOMYOSIN CYTOSKELETON OF AMEBAS OF THE CELLULAR SLIME-MOLDS ACRASIS-ROSEA AND PROTOSTELIUM-MYCOPHAGA - STRUCTURE, BIOCHEMICAL-PROPERTIES, AND FUNCTION

In amoebae of the cellular slime molds (mycetozoans) Acrasis rosea and Protostelium mycophaga, bundles of F-actin radiate from the endoplasm -ectoplasm interface into the pseudopodia, where G-actin is also locat ed. We conclude that these actin bundles form a core scaffold driving pseudopod extension which is subsequently completed by filling with a more loosely organized meshwork of F-actin. Some bipolar, elongate amo ebae of A. rosea also contained long bundles of F-actin that traverse the cells lengthwise and remotely resemble stress fibers. Rodlets of F -actin were scattered in the body of amoebae of A. rosea or formed sta r-shaped or polygonal complexes near or around contractile vacuoles, w here they may play a role in contraction. In total protein extracts an alyzed by SDS-PAGE and immunoblots the actins migrated like the rabbit skeletal muscle control. The relative proportion of actin in total pr otein extracts was 7.9% for A. rosea and 34.5% for P. mycophaga. We de tected four or five isoactins in extracts of both species and we deter mined that the genome of each species contains approximately six actin genes. Whether they are all expressed or if posttranslational modific ations occur remains to be determined. Myosin II was enriched in actom yosin extracts; its M-r was 187.8 kDa for A. rosea and 220.7 kDa for P . mycophaga. Cell models (''ghosts'') contracted upon the addition of ATP. We conclude that amoebae of A. rosea and P. mycophaga, although b ehaving differently from those of Dictyostelium discoideum, contain th e basic repertoire of molecules that enable pseudopod extension by act in polymerization and ATP-induced contraction of the cell cortex. (C) 1998 Academic Press.