FhCaBP2: a Fasciola hepatica calcium-binding protein with EF-hand and dynein light chain domains

FhCaBP2 is a Fasciola hepatica protein which belongs to a family of helminth calcium-binding proteins which combine an N-terminal domain containing two EF-hand motifs and a C-terminal dynein light chain-like (DLC-like) domain. Its predicted structure showed two globular domains joined by a flexible linker. Recombinant FhCaBP2 interacted reversibly with calcium and manganese ions, but not with magnesium, barium, strontium, copper (II), colbalt (II), iron (II), nickel, lead or potassium ions. Cadmium (II) ions appeared to bind non-site-specifically and destabilize the protein. Interaction with either calcium or magnesium ions results in a conformational change in which the protein's surface becomes more hydrophobic. The EF-hand domain alone was able to interact with calcium and manganese ions; the DLC-like domain was not. Alteration of a residue (Asp-58 to Ala) in the second EF-hand motif in this domain abolished ion-binding activity. This suggests that the second EF-hand is the one responsible for ion-binding. FhCaBP2 homodimerizes and the extent of dimerization was not affected by calcium ions or by the aspartate to alanine substitution in the second EF-hand. The isolated EF-hand and DLC-like domains are both capable of homodimerization. FhCaBP2 interacted with the calmodulin antagonists trifluoperazine, chlorpromazine, thiamylal and W7. Interestingly, while chlorpromazine and thiamylal interacted with the EF-hand domain (as expected), trifluoperazine and W7 bound to the DLC-like domain. Overall, FhCaBP2 has distinct biochemical properties compared with other members of this protein family from Fasciola hepatica, a fact which supports the hypothesis that these proteins have different physiological roles.