Molecular "tuning" of crystal growth by nacre-associated polypeptides

The formation and stabilization of the aragonite polymorph in the nacre layer of mollusks is an intriguing process, yet very little is known with regard to the participation of proteins in this process. Previously, we identified the 30 AA N-terminal mineral binding domains (AP7-N, AP24-N, n16-N) of three different nacre-specific proteins (AP7, AP24, n16). These three domains differ in primary sequence and induce morphological changes in CaCO3 crystals in vitro. Using AFM microscopy, we investigated the adsorption of AP7-N, AP24-N. and n16-N onto calcite dislocation hillocks. We observe that both AP7-N and AP24-N are multifunctional; they not only inhibit obtuse step advance but also induce rounded, amorphous-appearing deposits on hillock terraces. In contrast, n16-N pins corner sites at the junction of acute and obtuse steps and promote the emergence of a new set of steps approximately oriented along a line joining the obtuse-obtuse and the acute-acute corners. Random scrambling of the n16-N and AP7-N sequences resulted in substantially reduced mineral modification activities, indicating that the primary sequence of both polypeptides is crucial for correct recognition of surface features. These findings indicate that nacre proteins evolved specialized mineral interaction domains that either recognize different surface features (A-P7-N, AP24-N versus n16-N), or recognize the same features, but with different binding and catalytic activities (AP24-N versus AP7-N). These functional differences may arise from the differences in primary and secondary structure specific to each N-terminal domain. Hypothetically, this molecular diversity would allow complementary and simultaneous protein control ("molecular tuning") of different features of the crystal growth process.