Evidence for a Ca(2+)-specific conformational change in avian thymic hormone, a high-affinity beta-parvalbumin

Named for the capacity to stimulate differentiation and maturation of T-cell precursors, avian thymic hormone (ATH) is nonetheless a beta-parvalbumin that is also expressed in the avian retina. With Ca(2+)- and Mg(2+)-binding constants in excess of 10(8) and 10(4) M(-1), respectively, both EF-hand motifs qualify as Ca(2+)/Mg(2+) sites. However, whereas addition of either apo- or Mg(2+)-bound ATH to 1,8-anilinonaphthalenesulfonic acid (ANS) causes a large increase in quantum yield and a pronounced blue shift, addition of the Ca(2+)-bound protein is without effect. These observations suggest that apo- and Mg(2+)-bound ATH adopt conformations distinct from the Ca(2+)-bound protein, exposing apolar surface for interaction with ANS. Differential scanning calorimetry (DSC) data imply that unfolding of apo-ATH is accompanied by diminished exposure of apolar surface, relative to Ca(2+)-free rat beta-PV, perhaps due to greater solvent-accessible apolar surface in the native form. The fluorescence and DSC results, considered together, may indicate that the AB and CD-EF domains of ATH are not tightly associated in the absence of bound Ca(2+). Consistent with this idea, sedimentation velocity data reveal that the apo- and Mg(2+)-bound forms of ATH show greater departures from spherical symmetry than the Ca(2+)-bound state. These findings suggest that a high-affinity binding signature does not require that the parvalbumin apo- and Ca(2+)-bound conformations be indistinguishable, as we have recently proposed. They also suggest that it is possible to engineer a Ca(2+)-dependent conformational change into a high-affinity EF-hand protein, furnishing a mechanism by which the protein could play a reverse Ca(2+) sensor role.