Calcium regulation of actin crosslinking is important for function of the actin cytoskeleton in Dictyostelium

The actin cytoskeleton is sensitive to changes in calcium, which affect contractility, actin-severing proteins, actin-crosslinking proteins and calmodulin-regulated enzymes. To dissect the role of calcium control on the activity of individual proteins from effects of calcium on other processes, calcium-insensitive forms of these proteins were prepared and introduced into living cells to replace a calcium-sensitive form of the same protein. Crosslinking and bundling of actin filaments by the Dictyostelium 34 kDa protein is inhibited in the presence of micromolar free calcium. A modified form of the 34 kDa protein with mutations in the calcium binding EF hand (34 kDa DeltaEF2) was prepared using site-directed mutagenesis and expressed in E. coli. Equilibrium dialysis using [Ca-45]CaCl2 revealed that the wild-type protein is able to bind one calcium ion with a Kd of 2.4 muM. This calcium binding is absent in the 34 kDa DeltaEF2 protein. The actin-binding activity of the 34 kDa DeltaEF2 protein was equivalent to wildtype but calcium insensitive in vitro. The wild-type and 34 kDa DEF2 proteins were expressed in 34-kDa-null and 34 kDa/alpha-actinin double null mutant Dictyostelium strains to test the hypothesis that calcium regulation of actin-crosslinking is important in vivo. The 34 kDa DEF2 failed to supply function of the 34 kDa protein important for control of cell size and for normal growth to either of these 34-kDa-null strains. Furthermore, the distribution of the 34 kDa protein and actin were abnormal in cells expressing 34 kDa DEF2. Thus, calcium regulation of the formation and/or dissolution of crosslinked actin structures is required for dynamic behavior of the actin cytoskeleton important for cell structure and growth.