Originally isolated on the basis of its capacity to stimulate T-cell maturation and proliferation, avian thymic hormone (ATH) is nevertheless a parvalbumin, one of two β-lineage isoforms expressed in birds. We recently learned that addition of Ca2+-free ATH to a solution of 8-anilinonaphthalene-1-sulfonate (ANS) markedly increases ANS emission. This behavior, not observed in the presence of Ca2+, suggests that apolar surface area buried in the Ca2+-bound state becomes solvent accessible upon Ca2+ removal. In order to elucidate the conformational alterations that accompany Ca2+ binding, we have obtained the solution structure of the Ca2+-free protein using NMR spectroscopy and compared it to the Ca2+-loaded... More
Originally isolated on the basis of its capacity to stimulate T-cell maturation and proliferation, avian thymic hormone (ATH) is nevertheless a parvalbumin, one of two β-lineage isoforms expressed in birds. We recently learned that addition of Ca2+-free ATH to a solution of 8-anilinonaphthalene-1-sulfonate (ANS) markedly increases ANS emission. This behavior, not observed in the presence of Ca2+, suggests that apolar surface area buried in the Ca2+-bound state becomes solvent accessible upon Ca2+ removal. In order to elucidate the conformational alterations that accompany Ca2+ binding, we have obtained the solution structure of the Ca2+-free protein using NMR spectroscopy and compared it to the Ca2+-loaded protein, solved by X-ray crystallography. Although the metal-ion-binding (CD–EF) domains are largely coincident in the superimposed structures, a major difference is observed in the AB domains. The tight association of helix B with the E and F helices in the Ca2+-bound state is lost upon removal of Ca2+, producing a deep hydrophobic cavity. The B helix also undergoes substantial rotation, exposing the side chains of F24, Y26, F29, and F30 to solvent. Presumably, the increase in ANS emission observed in the presence of unliganded ATH reflects the interaction of these hydrophobic residues with the fluorescent probe. The increased solvent exposure of apolar surface area in the Ca2+-free protein is consistent with previously collected scanning calorimetry data, which indicated an unusually low change in heat capacity upon thermal denaturation. The Ca2+-free structure also provides added insight into the magnitude of ligation-linked conformational alteration compatible with a high-affinity metal-ion-binding signature. The exposure of substantial apolar surface area suggests the intriguing possibility that ATH could function as a reverse Ca2+ sensor.
