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TheJapanese pearl oyster (Pinctada fucata) n16 framework matrix protein is an integral part of the growthand formation of the mollusk shell biomineralization mechanism. Itis a required component of the extracellular matrix with a dual mineralizationrole, as an anchor component to synchronize the assembly of the beta-chitinand N-series, Pif-series protein extracellular matrix for aragoniteformation and as a regulator of aragonite formation itself. However,the mechanism by which this protein controls aragonite formation isnot understood. Here, we investigate the mineralization potentialand kinetics of the 30 AA N-terminal portion of the n16 protein, n16N.This sequence has been demonstrated to form either vaterite or aragonitedepending upon conditions. Using in situ potentiometric titrationmethods, we find that n16N is indeed responsible for the self-assemblycharacteristics found in vivo and in vitro but is not involved withactive Ca2+binding or mineral nucleation processes. Uponthe basis of time- and peptide concentration-dependent sampling ofmineral deposits that form in solution, we find that n16N is responsiblefor controlling where mineralization occurs in bulk solution. Thisprotein sequence acts as a molecular spacer that organizes the mineralizationspace and promotes the formation of mineral constituents that containACC, vaterite, and aragonite. Without the concerted action of then16N assemblage, unregulated calcite formation occurs exclusively.Thus, the n16 protein provides the regulation needed to have the characteristicpolymorph, crystalline orientations, and related mechanical propertiesassociated to the microstructure of mollusk shells. [ABSTRACT FROM AUTHOR]