Hookworm activation-associated secreted proteins can be structurally classified into at least three different groups. The hallmark feature of Group 1 activation-associated secreted proteins is a prominent equatorial groove, which is inferred to form a ligand binding site. Furthermore, a conserved tandem histidine motif is located in the centre of the groove and believed to provide or support a yet to be determined catalytic activity. Here, we report three-dimensional crystal structures of Na-ASP-2, an L3-secreted activationassociated secreted protein from the human hookworm Necator americanus, which demonstrate transition metal binding ability of the conserved tandem histidine motif. We further identified moderate phosphohydrolase activity of recombinant Na-ASP-2, which relates to the tandem histidine motif. By panning a random 12-mer peptide phage library, we identified a peptide with high similarity to the human calcium-activated potassium channel SK3, and confirm binding of the synthetic peptide to recombinant Na-ASP-2 by differential scanning fluorimetry. Potential binding modes of the peptide to Na-ASP-2 were studied by molecular dynamics simulations which clearly identify a preferred topology of the Na-ASP-2:SK3 peptide complex. KeywordsActivation-associated secreted proteins, Host-parasite interactions, Pathogenesis-related proteins, Protein structure, SCP/TAPS proteins Database Coordinates and structure factors have been deposited with the PDB, accession numbers 4nui, 4nuk, 4nun and 4nuo.

Mycobacterial carbonic anhydrases, such as the essential protein Rv3588c, are attractive drug targets since they constitute a different class of carbonic anhydrases than those found in humans.