Targeted a therapy holds tremendous potential as a cancer treatment: it offers the possibility of delivering a highly cytotoxic dose to targeted cells while minimizing damage to surrounding healthy tissue. The metallic alpha-generating radioisotopes Ac-225 and Th-227 are promising radionuclides for therapeutic use, provided adequate chelation and targeting. Here we demonstrate a new chelating platform composed of a multidentate high-affinity oxygen-donating ligand 3,4,3-LI(CAM) bound to the mammalian protein siderocalin. Respective stability constants log beta(110) = 29.65 +/- 0.65, 57.26 +/- 0.20, and 47.71 +/- 0.08, determined for the Eu-III (a lanthanide surrogate for Ac-III), Zr-IV, and Th-IV complexes of 3,4,3-LI(CAM) through spectrophotometric titrations, reveal this ligand to be one of the most powerful chelators for both trivalent and tetravalent metal ions at physiological pH. The resulting metal ligand complexes are also recognized with extremely high affinity by the siderophore-binding protein siderocalin, with dissociation constants below 40 nM and tight electrostatic interactions, as evidenced by X-ray structures of the protein:ligand:metal adducts with Zr-IV and Th-IV. Finally, differences in biodistribution profiles between free and siderocalin-bound Pu-238(IV)-3,4,3-LI(CAM) complexes confirm in vivo stability of the protein construct. The siderocalin:3,4,3-LI(CAM) assembly can therefore serve as a "lock" to consolidate binding to the therapeutic Ac-225 and Th-227 isotopes or to the positron emission tomography emitter (89) Zr, Zr, independent of metal valence state.