The N- and C-terminal regions of the pearl-associated EF hand protein, PFMG1, promote the formation of the aragonite polymorph in vitro

Recent studies indicate that the ability of mollusk shell nacre protein sequences to form the calcium carbonate polymorph, aragonite, are linked to the presence of intrinsically disordered sequences within these proteins. Although the exact relationship between protein structural disorder and polymorph formation is not clear, there is a definite interest in discovering other examples of intrinsically disordered nacre protein sequences that can induce aragonite formation. In this report, we extend the relationship between intrinsic disorder and aragonite formation to another set of nacre protein sequences. This protein, known as PFMG1, is associated with pearl formation in the Japanese pearl oyster, Pinctada fucata. We demonstrate that synthetic peptides representing the 30 AA N- and C-terminal sequence regions of PFMG1 nucleate nanoscale-sized aragonite in solution without the need for additional additives. Compared to controls containing no peptide or bovine serum albumin, the PFMG1 terminal sequences appear to form a matrix-like environment around the forming biominerals, and this process will be defined in more detail in later reports. Furthermore, we establish that these PFMG1 terminal sequences possess disordered structures in solution that can be stabilized into partially folded structures (α helix, beta structures) using the structure-stabilizing solvent, 2,2,2-trifluoroethanol. Although we do not know the mechanism by which these peptides promote aragonite nucleation in vitro, we believe that these terminal sequences are participants in PFMG1-mediated aragonite polymorph formation within the oyster pearl and that the intrinsic disorder and folding propensities of these sequences are crucial for this activity.