Reversible, β-sheet-dependent self-assembly of the phosphoprotein phosvitin is controlled by the concentration and valency of cations

Abstract

The hyperphosphorylated protein phosvitin (PV) undergoes a pH-dependent transition between P_II⁻ and β-sheet secondary structures, a process deemed crucial for its role in the promotion of biogenic apatite formation. The transition occurs surprisingly slowly (minutes to hours). This is consistent with a slow aggregation process involving ionic interactions of charged groups on the protein surface. Herein, we determined the associated transition pK values and time constants through matrix least-squares (MLS) global fitting of a series of pH- and time-dependent circular dichroism (CD) spectra recorded in the presence of different mono-, bi- and trivalent cations. Supporting our results with dynamic light scattering data, we clearly identified a close correlation of β-sheet transition and the formation of small aggregates at low pH. This process is inhibited in the presence of all tested cations with the strongest effects for trivalent cations (Fe³⁺ and Al³⁺). In the presence of Ca²⁺ and Mg²⁺, larger higher-order particles are formed from PV in the β-sheet conformation, as identified from the interpretation of differential scattering observed in the CD spectra. Our observations are consistent with the existence of a multi-step equilibrium between aggregated and non-aggregated species of PV. The equilibrium is highly sensitive to the environment pH and salt concentration with exceptional behavior in the presence of divalent cations such as Ca²⁺ and Mg²⁺.