Extremely charged coral protein AGARP regulates calcium carbonate growth through liquid phase separation
Biomineralization via the non-classical crystallization pathway is postulated to involve a transient liquid phase of calcium carbonate formed in the presence of polymers. In the context of coral biocalcification, these polymers may include coral acid-rich proteins (CARPs) secreted into the skeletal organic matrix. However, direct evidence for the existence of this liquid phase containing proteins is lacking. Here, we show that the intrinsically disordered aspartic and glutamic acid-rich protein (AGARP), the first CARP cloned from the Great Barrier Reef scleractinian coral Acropora millepora, can significantly influence early stages of CaCO3 nucleation and crystal growth through liquid-liquid phase separation. We introduce the concept of a biologically relevant crystallization precursor: a liquid protein-calcium condensate composed of CARP molecules and Ca2+ ions, which forms as a result of liquid-liquid phase separation in a crowded environment. Our work bridges the gap between the liquid phase separation and biomineralization research.