Three actin cross-linking proteins, the 34 kDa actin-bundling protein, alpha-actinin and gelation factor (ABP-120), have both unique and redundant roles in the growth and development of Dictyostelium.

The contribution of three actin cross-linking proteins, alpha-actinin (alphaA), gelation factor (ABP-120), and the 34 kDa actin-bundling protein to cellular functions has been studied in three single mutant (alphaA-, 120-, and 34-) and three double mutant (alphaA-/120-, 34-/alphaA-, 34-/120-) strains of Dictyostelium generated by homologous recombination. Strains alphaA-/120- and 34-/alphaA- exhibited a reduced rate of pinocytosis, grew to lower saturation densities, and produced small cells in shaking cultures. All strains grew normally in bacterial suspensions and on agar plates with a bacterial lawn. Slow growth under conditions of reduced temperature and increased osmolarity was observed in single mutants 34- and alphaA-, respectively, as well as in some of the double mutant strains. Motility, chemotaxis, and development were largely unaltered in 34-/alphaA- and 34-/120- cells. However, 34-/alphaA- cells showed enhanced aggregation when starved in suspension. Moreover, morphogenesis was impaired in both double mutant strains and fruiting bodies of aberrant morphology were observed. These defects were reverted by re-expression of one of the lacking cross-linking proteins. The additive and synthetic phenotypes of these mutations indicate that actin cross-linking proteins serve both unique and overlapping functions in the actin cytoskeleton.