The Actomyosin Cytoskeleton of Amoebae of the Cellular Slime MoldsAcrasis roseaandProtostelium mycophaga: Structure, Biochemical Properties, and Function

Abstract

In amoebae of the cellular slime molds (mycetozoans)Acrasis roseaandProtostelium mycophaga, bundles of F-actin radiate from the endoplasm–ectoplasm interface into the pseudopodia, where G-actin is also located. 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 amoebae ofA. roseaalso 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 ofA. roseaor formed star-shaped or polygonal complexes near or around contractile vacuoles, where they may play a role in contraction. In total protein extracts analyzed by SDS–PAGE and immunoblots the actins migrated like the rabbit skeletal muscle control. The relative proportion of actin in total protein extracts was 7.9% forA. roseaand 34.5% forP. mycophaga. We detected four or five isoactins in extracts of both species and we determined that the genome of each species contains approximately six actin genes. Whether they are all expressed or if posttranslational modifications occur remains to be determined. Myosin II was enriched in actomyosin extracts; itsMrwas 187.8 kDa forA. roseaand 220.7 kDa forP. mycophaga. Cell models (“ghosts”) contracted upon the addition of ATP. We conclude that amoebae ofA. roseaandP. mycophaga, although behaving differently from those ofDictyostelium discoideum, contain the basic repertoire of molecules that enable pseudopod extension by actin polymerization and ATP-induced contraction of the cell cortex.