An extended hudrophobic core induces EF‐hand swapping

The structure of calbindin D_9k with two substitutions was determined by X‐ray crystallography at 1.8‐Å resolution. Unlike wild‐type calbindin D_9k, which is a monomeric protein with two EF‐hands, the structure of the mutated calbindin D_9k reveals an intertwined dimer. In the dimer, two EF‐hands of the monomers have exchanged places, and thus a 3D domain‐swapped dimer has been formed. EF‐hand I of molecule A is packed toward EF‐hand II of molecule B and vice versa. The formation of a hydrophobic cluster, in a region linking the EF‐hands, promotes the conversion of monomers to 3D domain‐swapped dimers. We propose a mechanism by which domain swapping takes place via the apo form of calbindin D_9k. Once formed, the calbindin D_9k dimers are remarkably stable, as with even larger misfolded aggregates like amyloids. Thus calbindin D_9k dimers cannot be converted to monomers by dilution. However, heating can be used for conversion, indicating high energy barriers separating monomers from dimers.