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The F-actin crosslinking molecule alpha-actinin from the slime mould Dictyostelium discoideum carries two characteristic EF-hand structures at the C-terminus. The calcium-binding loops contain all necessary liganding oxygens and most likely form the structural basis for the calcium sensitivity of strictly calcium-regulated non-muscle alpha-actinins. Furthermore, the sequence exhibits at the N-terminal site of the molecule a high degree of homology to chicken fibroblast alpha-actinin. This stretch of amino acids appears to have remained essentially constant during evolution and might represent the actin-binding site. The findings have led us to propose a model for the inhibitory action of Ca2+ on non-muscle alpha-actinins.