Oligomerization of an avian thymic parvalbumin. Chemical evidence for a Ca(2+)-specific conformation

CPV3, the third parvalbumin isoform to be identified in the chicken, is produced exclusively in the thymus gland. Although parvalbumins are typically cysteine-deficient, CPV3 contains two cysteine residues, at positions 18 and 72. The reported three-dimensional parvalbumin structures suggest that the side chain of cysteine-72 should be solvent-accessible. Accordingly, we find that CPV3 readily forms disulfide-linked oligomers in the absence of reducing agents. The reaction, employing either O2 or ferricyanide ion as the oxidant, is apparently restricted to the Ca(2+)-bound form of the protein. The differing reactivity of the Ca2+, Mg2+, and apo-forms has significant structural implications.