Structure of Ca2+-loaded human grancalcin

Grancalcin is a cytosolic Ca2+-binding protein originally identified in hum an neutrophils. It belongs to a new class of EF-hand proteins, called PEF p roteins, which contain five EF-hand motifs. At the N-terminus of grancalcin there is a similar to 50 residue-long segment rich in glycines and proline s. The fifth EF-hand, unpaired within the monomer, provides a means for dim erization through pairing with its counterpart in a second molecule. The st ructure of full-length grancalcin in the apo form and with one EF3 within t he dimer occupied by a Ca2+ ion have been determined. Although the N-termin al segment was present in the molecule, this part was disordered in the cry stals. Here, the structure of a truncated form of grancalcin, which is lack ing 52 N-terminal residues, in the presence and absence of Ca2+ is presente d. In the Ca2+-bound form the ions are found in the EF1 and EF3 hands. Bind ing of Ca2+ to these two EF hands produces only minor conformational change s, mostly within the EF1 Ca2+-binding loop. This observation supports the h ypothesis, formulated on the basis of the structure of a homologous protein ALG-2 which shows significant differences in the orientation of EF4 and EF 5 compared with grancalcin, that calcium is a necessary factor but not suff icient alone for inducing a significant conformational change in PEF protei ns.