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An oligomeric C-RING nacre protein influences pre-nucleation events and organizes mineral nanoparticles

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JournalBiochemistry
DateE-pub ahead of print - 29 Oct 2014
DatePublished (current) - Nov 2014
Issue number46
Volume53
Pages (from-to)7259–7268
Early online date29/10/14
Original languageEnglish

Abstract

The mollusk shell nacre layer integrates mineral phases with macromolecular components such as intracrystalline proteins. However, the roles performed by these proteins in the early events of calcium carbonate nucleation and subsequent post-nucleation events (e.g., organization of mineral deposits) in the nacre layer are not known. We find that AP7, a nacre intracrystalline CRING protein, self-assembles to form amorphous protein oligomers and films on mica that further assemble into larger aggregates or phases in the presence of Ca2+. Using solution NMR spectroscopy we determine that the protein assemblies are stabilized by interdomain interactions involving the aggregation-prone T31-N66 C-terminal C-RING domain but are destabilized by the labile nature of the intrinsically disordered D1-T19 AA N-terminal sequence. Thus the dynamic, amorphous nature of the AP7 assemblies is traceable to the molecular behavior of the N-terminal sequence. Using potentiometric methods we observe that AP7 protein phases prolong the time interval for pre-nucleation cluster formation, but neither stabilize nor destabilize ACC clusters. Time-resolved flow cell STEM mineralization studies confirm that AP7 protein phases delay the onset of nucleation and assemble and organize mineral nanoparticles into ring-shaped branching clusters in solution. These phenomena are not observed in protein-deficient assays. We conclude that C-RING AP7 protein phases modulate the time period for early events in nucleation and form strategic associations with forming mineral nanoparticles that lead to mineral organization.

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