Temperature Affects the Supramolecular Structures Resulting from α-Lactalbumin−Lysozyme Interaction

Abstract

The interaction between alpha-lactalbumin and lysozyme, two globular proteins with highly homologous tertiary structures but opposite electric charges, was investigated. As assessed by isothermal titration calorimetry, lysozyme did not bind to the native form of alpha-lactalbumin but did interact with calcium-depleted alpha-lactalbumin (apo alpha-LA). This interaction leads to the formation of different supramolecular structures depending on the temperature. Heterogeneous, amorphous aggregates are formed at 5 degrees C, while droplets, coacervate-like structures, exist at 45 degrees C. The coacervates are formed by equimolar quantities of the two proteins, but their size and number depend on the initial protein molar ratio. These supramolecular structures are found to be stable when the temperature is decreased to 5 degrees C, while prolonged heating at 45 degrees C induces the formation of larger coacervates through a coalescence phenomenon. Surprisingly, interplay occurs between aggregates and coacervates when the temperature is increased from 5 to 45 degrees C. We discuss the results in terms of subtle heat-induced conformational changes in apo alpha-LA. In conclusion, our results show an association between globular proteins that leads to the formation of a variety of supramolecular structures in a temperature-dependent manner and confirm the primordial role of certain alpha-lactalbumin unfolding intermediates in protein-driven assembly.