A nacre protein, n16.3, self-assembles to form protein oligomers that dimensionally limit and organize mineral deposits.
The mollusk shell is a complex biological material that integrates mineral phases with organic macromolecular components such as proteins. The role of proteins in the formation of the nacre layer (aragonite mineral phase) is poorly understood, particularly with regard to the organization of mineral deposits within the protein extracellular matrix and the identification of which proteins are responsible for this task. We report new experiments that provide insight into the role of the framework nacre protein, n16.3 (Pinctada fucata), as an organizer or assembler of calcium carbonate mineral clusters. Using a combination of biophysical techniques, we find that recombinant n16.3 (r-n16.3) oligomerizes to form amorphous protein films and particles that possess regions of disorder and mobility. These supramolecular assemblies possess an intrinsically disordered C-terminal region (T64-W98) and reorganize in the presence of Ca(2+) ions to form clustered protein oligomers. This Ca(2+)-induced reorganization leads to alterations in the molecular environments of Trp residues, the majority of which reside in putative aggregation-prone cross-β strand regions. Potentiometric Ca(2+) titrations reveal that r-n16.3 does not significantly affect the formation of prenucleation clusters in solution, and this suggests a role for this protein in postnucleation mineralization events. This is verified in subsequent in vitro mineralization assays in which r-n16.3 demonstrates its ability to form gel-like protein phases that organize and cluster nanometer-sized single-crystal calcite relative to protein-deficient controls. We conclude that the n16 nacre framework proteome creates a protein gel matrix that organizes and dimensionally limits mineral deposits. This process is highly relevant to the formation of ordered, nanometer-sized nacre tablets in the mollusk shell.
Laboratory for Chemical Physics, Division of Basic Sciences and Craniofacial Biology, New York University , 345 East 24th Street, New York, New York 10010, United States.
Iva
I
Perovic
Eric P
EP
Chang
Michael
M
Lui
Ashit
A
Rao
Helmut
H
Cölfen
John Spencer
JS
Evans
D000818:Animals; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D002119:Calcium Carbonate / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D018625:Microscopy, Atomic Force; D060734:Nacre / Q000737:chemistry*; D049911:Pinctada / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D011506:Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000737:chemistry
D060734:Nacre; D011506:Proteins; D014364:Tryptophan; D002119:Calcium Carbonate; D002118:Calcium