The formation of the nacre pearl in marine invertebrates represents an on-demand production of mineralization in response to an irritant or parasite threat to the mantle organ. In the Japanese pearl oyster (Pinctada fucata), this process is mediated by a 12-member protein family known as PFMG (Pinctada fucata mantle gene). One of these proteins, PFGM1, has been implicated in modulating calcium carbonate crystal growth and has been reported to possess an EF-hand-like domain. In this report, we establish that the recombinant PFMG1 (rPFMG1) is an intrinsically disordered "imitator" EF-hand protein that increases the number of calcium carbonate mineral crystals that form relative to control scenarios and does not induce aragonite formation. This protein possesses a modified pseudo-EF-hand sequence at the C-terminal end which exhibits low homology (30-40%) to the pseudo-EF-hand mitochondrial SCaMCs buffering/solute transport proteins. This low sequence homology is the result of the inclusion of disorder-promoting amino acids and short amyloid-like aggregation-prone cross-β-strand sequences within the putative PFMG1 pseudo-EF-hand sequence region. Similar to other nacre proteins, rPFMG1 oligomerizes to form amorphous, heterogeneously sized protein oligomers and films in vitro, and this process is enhanced by Ca2+, which promotes the formation of aggregation-prone extended β-strand structure within rPFMG1. From these results, we conclude that PFMG1 forms supramolecular assemblies that play an important role in amplifying the nucleation process that is crucial for coating or neutralizing invasive threats to the mantle organ.
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