Polymorph Crystal Selection by n16, an Intrinsically Disordered Nacre Framework Protein

Framework proteins are a subclass of mollusk shell nacre-, associated polypeptides that form supramolecular assemblies with beta-chitin and other matrix proteins. These macromolecular assemblies manage the energetics of aragonite polymorph nucleation, and thus, there is keen interest in under- standing the molecular characteristics of framework proteins. Here, we report the mineralization activity, oligomerization, and structural features of a recombinant.. framework nacre protein n16, isoform 3 (r-n16.3, Japanese pearl oyster Pinctada fucata). We find that r-n16.3 assembles in Mineralization solutions to form spheroidal-fibril and mineralized thin film assemblies, in addition to spherical vaterite mineral deposits and aragonite single crystal deposits that possess unusual texture and layered morphologies The oligomerization of r-n16.3 is spontaneous over the pH range 5-8.5, and protein particle sizes are observed to increase in radii when Ca(II) is present Bioinformatics studies revel that the r-n16.3 molecule is intrinsically disordered (random coil) and possesses residual alpha helix and beta sheet structure. Experimentally, we confirmed that the secondary structure of apo-r-n16.3 assemblies is largely disordered (50% random coil, 2096 p strand, 8% a helix). However, in the presence of high Ca(II) concentrations, we observe IDP disorder-to-order transformations that increase beta turn structure and decrease random coil, alpha helix, and beta strand contents. We conclude that r-n16.3 is an intrinsically disordered oligomeric nacre framework protein that nucleates vaterite and single crystal aragonite in vitro and possesses target specific IDP disorder-to-order transformation capabilities in response to Ca(II).