Figure 1

Immunization scheme of llamas with CDTa and CDTb.

(A) Schematic diagram of the toxin subunits used for immunization. CDTa and CDTb are independently produced as secreted proteins with an N-terminal signal peptide that is cleaved during secretion. CDTa consists of two tandem ADP-ribosyltransferase (ART) domains. The N-terminal ART domain does not possess enzymatic activity and mediates binding to CDTb; the C-terminal ART domain mediates ADP-ribosylation of actin at Arg 177 and contains a deep NAD⁺-binding cleft. Proteolytic cleavage of its N-terminal activation domain (PA14) allows CDTb to form oligomers that bind CDTa and assists its delivery to the host cell cytosol. 3D models were generated with PyMol using coordinates of the pdb file 2wn7 for CDTa with NAD⁺ (cyan). CDTb was modeled onto the 3D structure of iota toxin (pdb code 2j42) using SwissModel and assembled into the multimer by alignment with anthrax toxin (pdb code 3hvd) using PyMol. (B) Three llamas (Lama glama) were immunized subcutaneously with purified recombinant CDTa or CDTb emulsified with Specol adjuvant, boosted and bled as indicated in the time scale.