Modulation of crystal growth by the terminal sequences of the prismatic-associated asprich protein

The formation of calcite in the mollusk shell prismatic layer requires the participation of various proteins. Recent studies indicate that the prismatic-associated protein superfamily, Asprich, is capable of in vitro stabilization of amorphous calcium carbonate (ACC), a precursor phase of prismatic calcite. To learn more about the molecular behavior of Asprich, we performed experiments on two highly conserved sequences derived from Asprich: Fragment-1, a 48 AA N-terminal cationic-anionic sequence, and Fragment-2, a previously characterized 42 C-terminal AA anionic sequence. SEM analyses reveal that Fragment-1 induces polycrystalline, radial aggregate assemblies of calcite, with evidence of surface porosities. AFM flow cell experiments demonstrate that Fragment-1 is multifunctional and its mineralization behavior is qualitatively similar to that reported for Fragment-2 except for hillock step kinetics. Surprisingly, when Fragment-1 and Fragment-2 are present together within the same assay, we observe phase stabilization of vaterite on Kevlar substrates and amorphous-appearing islands on calcite substrates. We believe that island formation on the calcite substrate results from the deposition of peptide-mineral clusters onto calcite hillock terrace surfaces. These events may also take place on the Kevlar substrate as well, where either vaterite or calcite form. The most significant feature is that a mixture of Fragment-1 + Fragment-2 are required to induce these effects and that the individual sequences themselves do not have this capability. These results indicate that these conserved terminal Asprich sequences jointly exhibit mineralization behavior (i.e., phase stabilization) that is qualitatively similar to the parent protein, and, parallels the in vitro findings reported for other calcite and aragonite - associated polypeptide sequences. It is likely that the sequence features of Asprich may be used to design crystal growth mimetics that can modulate crystal growth within the laboratory setting.