Re-expression of ABP-120 rescues cytoskeletal, motility, and phagocytosis defects of ABP-120^- Dictyostelium mutants

The actin binding protein ABP-120 has been proposed to cross-link actin filaments in nascent pseudopods, in a step required for normal pseudopod extension in motile Dictyostelium amoebae. To test this hypothesis, cell lines that lack ABP-120 were created independently either by chemical mutagenesis or homologous recombination. Different phenotypes were reported in these two studies. The chemical mutant shows only a subtle defect in actin cross-linking, while the homologous recombinant mutants show profound defects in actin cross-linking, cytoskeletal structure, pseudopod number and size, cell motility and chemotaxis and, as shown here, phagocytosis. To resolve the controversy as to what the ABP-120 phenotype is, ABP-120 was re-expressed in an ABP-120 cell line created by homologous recombination. Two independently 'rescued' cell lines that express wild-type levels of ABP-120 were analyzed. In both rescued cell lines, actin incorporation into the cytoskeleton, pseudopod formation, cell morphology, instantaneous velocity, phagocytosis, and chemotaxis were restored to wild-type levels. There is no alteration in the expression levels of several related actin binding proteins in either the original ABP-120^- cell line or in the rescued cell lines, leading to the conclusion that neither the aberrant phenotype observed in ABP-120^- cells nor the normal phenotype reasserted in rescued cells can be attributed to alterations in the levels of other abundant and related actin binding proteins. Re-expression of ABP-120 in ABP-120^- cells reestablishes normal structural and behavioral parameters, demonstrating that the severity and properties of the structural and behavioral defects of ABP-120^- cell lines produced by homologous recombination are the direct result of the absence of ABP-120.