Structure of Ca²⁺-loaded human grancalcin

Grancalcin is a cytosolic Ca²⁺-binding protein originally identified in human neutrophils. It belongs to a new class of EF-hand proteins, called PEF proteins, which contain five EF-­hand motifs. At the N-terminus of grancalcin there is a ∼50 residue-long segment rich in glycines and prolines. The fifth EF-hand, unpaired within the monomer, provides a means for dimerization through pairing with its counterpart in a second molecule. The structure of full-length grancalcin in the apo form and with one EF3 within the dimer occupied by a Ca²⁺ ion have been determined. Although the N-terminal segment was present in the molecule, this part was disordered in the crystals. Here, the structure of a truncated form of grancalcin, which is lacking 52 N-terminal residues, in the presence and absence of Ca²⁺ is presented. In the Ca²⁺-bound form the ions are found in the EF1 and EF3 hands. Binding of Ca²⁺ to these two EF hands produces only minor conformational changes, mostly within the EF1 Ca²⁺-binding loop. This observation supports the hypothesis, formulated on the basis of the structure of a homologous protein ALG-2 which shows significant differences in the orientation of EF4 and EF5 compared with grancalcin, that calcium is a necessary factor but not sufficient alone for inducing a significant conformational change in PEF proteins.