The avian parvalbumin called CPV3 readily forms disulfide-linked oligomers. Sedimentation data presented herein reveal that CPV3 also undergoes noncovalent self-association. Interestingly, the noncovalent interaction is promoted by either Ca2+ or Mg2+, whereas covalent complex formation displays an absolute requirement for the Ca2+-bound protein. Apo-CPV3 exhibits an apparent sedimentation coefficient of 2.08 S at 20°C, in 0.15 M NaCl, 0.025 M HEPES-NaOH, pH 7.4. This value increases to 2.85 S or 3.16 S with addition of 1.0 mM Ca2+ or 5.0 mM Mg2+, respectively. Least-squares analysis of sedimentation equilibrium data suggests that 100 μM apo-CPV3 is primarily a mixture of monomeric and dimeric forms. With the addition of Ca2+, the equilibrium becomes exclusively monomer-trimer, with negligible amounts of dimer. A comparable distribution is observed in the presence of Mg2+.
