A plasma membrane-associated protein of Arabidopsis thaliana AtPCaP1 binds copper ions and changes its higher order structure

PCaP1, a hydrophilic cation-binding protein, is bound to the plasma membrane in Arabidopsis thaliana. We focused on the physicochemical properties of PCaP1 to understand its uniqueness in terms of structure and binding of metal ions. On fluorescence analysis, PCaP1 showed a signal of structural change in the presence of Cu2+. The near-UV CD spectra showed a marked change of PCaP1 in CuCl2 solution. The far-UV CD spectra showed the presence of alpha-helices and the intrinsically unstructured region. However, addition of Cu2+ gave no change in the far-UV CD spectra. These results indicate that Cu2+ induced a change in the tertiary structure without changing the secondary structure. The protein was sensitive to proteinase in the presence of Cu2+, supporting that Cu2+ is involved in the structural change. The PCaP1 solution was titrated with CuCl2 and the change in the fluorescence spectrum was monitored to characterize Cu2+-binding properties. The obtained values of K-d for Cu2+ and the ligand-binding number were 10 mu M and six ions per molecule, respectively. These findings indicate that PCaP1 has a high Cu2+-binding capacity with a relatively high affinity. PCaP1 lacks cysteine and histidine residues. A large number of glutamate residues may be involved in the Cu2+ binding.