Acellular slime mold,
Physarum polycephalum, has a unique wound-healing system. When cytoplasm of plasmodia is exposed to extracellular fluid, calcium binding protein 40 (CBP40) seals damaged areas, forming large aggregates in a Ca
2+ dependent manner. Part of the CBP40 is truncated at the N terminus by a proteinase in plasmodia (CBP40Δ), which does not aggregate in the Ca
2+-bound form. Here we report the crystal structures of CBP40Δ in both the metal-free and the Ca
2+-bound states. Both structures consist of three domains : coiled-coil, intervening, and EF-hand. The topology of the EF-hand domain is similar to that of calpain. The N-terminal half of CBP40Δ interacts with the C-terminal EF-hands through a large hydrophobic interface, necessary for high Ca
2+ affinity. Conformational change upon Ca
2+ binding is small ; however, the structure of CBP40Δ provides novel insights into the mechanism of Ca
2+-dependent oligomerization.
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