A pearl protein self-assembles to form protein complexes that amplify mineralization

Area of Science:

• Biomineralization
• Marine Invertebrate Biology
• Protein Chemistry

Background:

• Nacre pearl formation in marine invertebrates is a defense mechanism against irritants.
• The Pinctada fucata mantle gene (PFMG) family, including PFMG1, mediates this biomineralization process.
• PFMG1 is known to influence calcium carbonate crystal growth and has an EF-hand-like domain.

Purpose of the Study:

• To characterize the recombinant PFMG1 (rPFMG1) protein.
• To understand its role in calcium carbonate nucleation and crystal formation.
• To investigate the structural properties and self-assembly behavior of rPFMG1.

Main Methods:

• Production and characterization of recombinant PFMG1 (rPFMG1).
• Assays to measure calcium carbonate crystal formation and nucleation.
• In vitro studies on protein oligomerization and film formation, including Ca(2+) effects.
• Sequence analysis of the pseudo-EF-hand domain.

Main Results:

• rPFMG1 is an intrinsically disordered protein that enhances calcium carbonate crystal nucleation without inducing aragonite.
• It possesses a modified pseudo-EF-hand with low homology to other proteins, featuring disorder-promoting and aggregation-prone sequences.
• rPFMG1 oligomerizes into amorphous structures, with Ca(2+) enhancing aggregation and beta-strand formation.

Conclusions:

• PFMG1 forms supramolecular assemblies that amplify nucleation, a key process for neutralizing threats to the mantle organ.
• The protein's disordered nature and self-assembly properties are critical for its function in biomineralization.
• PFMG1's unique structure contributes to its role in the defense and pearl formation mechanisms of the Japanese pearl oyster.