Structural characterization of the N-terminal mineral modification domains from the molluscan crystal-modulating biomineralization proteins, AP7 and AP24

Temperature elevation and acidification damage microstructure of abalone via expression change of crystal induction genes

Ocean warming and acidification caused by global climate change interferes with the shell growth of mollusks. In abalone Haliotis discus hannai, the microstructural changes in the shell under stress are unclear, and the effect of thermal stress on biomineralization is unknown. The lack of gene information has also hampered the study of abalone biomineralization mechanisms. In this study, the microstructure of reconstructed shell in H. discus hannai was observed to determine the effects of thermal and acidification stress on shell growth. Three nacre protein genes, Hdh-AP7, Hdh-AP24, and Hdh-perlustrin, were characterized, and their expression pattern during shell repair was measured under thermal and acidification stress and compared with those of two known biomineralization-related genes, Hdh-AP-1 and Hdh-defensin. The stress resulted in aragonite plates with corroded or irregular microstructures. The gene expression of two nacre proteins (Hdh-AP7 and Hdh-AP24), which directly induce crystal formation, were more sensitive to thermal stress than to acidification, but the expression of the regulatory nacre protein (Hdh-perlustrin) and the two known genes (Hdh-AP-1 and Hdh-defensin), which are also related to immunity, showed an interlinked, complex pattern change. We concluded that high temperature and acidification damages the shell microstructure by disturbing the expression pattern of biomineralization-related genes.

A novel matrix protein PfX regulates shell ultrastructure by binding to specific calcium carbonate crystal faces

Here, we have identified a novel matrix protein, named PfX, from the pearl oyster Pinctada fucada, and investigated the effects of recombinant PfX protein on calcium carbonate crystallization. The expression of PfX was spatially concentrated in the mantle tissue and gill, the former of which is responsible for the formation of shell structures. The shell notching assay showed a PfX expression response during injured shell repair and regeneration, suggesting the potential involvement of this matrix protein in shell biomineralization. Further, an in vitro crystallization assay showed that PfX could alter the CaCO₃ morphologies of both calcite and aragonite polymorphs. Correspondingly, a binding assay indicated that PfX has strong binding affinity for CaCO₃ crystals, especially aragonite. Further, the protein's calcite binding capacity increased obviously when particular crystal faces were induced. In addition, PfX conjugated with fluorescent dye cyanine-5 (cy5) was preferentially distributed on rough crystal faces instead of the smooth and common (1 0 4) faces of calcite during the crystallization. These results suggest that matrix protein PfX might regulate CaCO₃ morphology via selective binding and inhibit the growth of certain crystal faces, providing new clues for understanding biomineralization mechanisms in mollusk.

A Novel Matrix Protein, PfY2, Functions as a Crucial Macromolecule during Shell Formation

Biomineralization, including shell formation, is dedicatedly regulated by matrix proteins. PfY2, a matrix protein detected in the ethylene diamine tetraacetic acid (EDTA)-soluble fraction from both prismatic layer and nacreous layer, was discovered by our group using microarray. It may play dual roles during biomineralization. However, the molecular mechanism is still unclear. In this research, we studied the function of PfY2 on crystallization in vivo and in vitro, revealing that it might be a negative regulator during shell formation. Notching experiment indicated that PfY2 was involved in shell repairing and regenerating process. Repression of PfY2 gene affected the structure of prismatic and nacreous layer simultaneously, confirming its dual roles in shell formation. Recombinant protein rPfY2 significantly suppressed CaCO₃ precipitation rate, participated in the crystal nucleation process, changed the morphology of crystals and inhibited the transformation of amorphous calcium carbonate (ACC) to stable calcite or aragonite in vitro. Our results may provide new evidence on the biomineralization inhibition process.

Design strategies and applications of nacre-based biomaterials

The field of tissue engineering and regenerative medicine relies heavily on materials capable of implantation without significant foreign body reactions and with the ability to promote tissue differentiation and regeneration. The field of bone tissue engineering in particular requires materials capable of providing enhanced mechanical properties and promoting osteogenic cell lineage commitment. While bone repair has long relied almost exclusively on inorganic, calcium phosphate ceramics such as hydroxyapatite and their composites or on non-degradable metals, the organically derived shell and pearl nacre generated by mollusks has emerged as a promising alternative. Nacre is a naturally occurring composite material composed of inorganic, calcium carbonate plates connected by a framework of organic molecules. Similar to mammalian bone, the highly organized microstructure of nacre endows the composite with superior mechanical properties while the organic phase contributes to significant bioactivity. Studies, both in vitro and in vivo, have demonstrated nacre's biocompatibility, biodegradability, and osteogenic potential, which are superior to pure inorganic minerals such as hydroxyapatite or non-degradable metals. Nacre can be used directly as a bulk implant or as part of a composite material when combined with polymers or other ceramics. While nacre has demonstrated its effectiveness in multiple cell culture and animal models, it remains a relatively underexplored biomaterial. This review introduces the formation, structure, and characteristics of nacre, and discusses the present and future uses of this biologically-derived material as a novel biomaterial for orthopedic and other tissue engineering applications.

STATEMENT OF SIGNIFICANCE

Mussel derived nacre, a biological composite composed of mineralized calcium carbonate platelets and interplatelet protein components, has recently gained interest as a potential alternative ceramic material in orthopedic biomaterials, combining the integration and mechanical capabilities of calcium phosphates with increased bioactivity derived from proteins and biomolecules; however, there is limited awareness of this material's potential. Herein, we present, to our knowledge, the first comprehensive review of nacre as a biomaterial. Nacre is a highly promising yet overlooked biomaterial for orthopedic tissue engineering with great potential in a wide variety of material systems. It is our hope that publication of this article will lead to increased community awareness of the potential of nacre as a versatile, bioactive ceramic capable of improving bone tissue regeneration and will elicit increased research effort and innovation utilizing nacre.

Keystone predation and molecules of keystone significance

Keystone species structure ecological communities and are major determinants of biodiversity. A synthesis of research on keystone species is nonetheless missing a critical component - the sensory mechanisms for behavioral interactions that determine population- and community-wide attributes. Here, we establish the chemosensory basis for keystone predation by sea stars (Pisaster ochraceus) on mussels. This consumer-resource interaction is prototypic of top-down driven trophic cascades. Each mussel species (Mytilus californianus and M. galloprovincialis) secretes a glycoprotein orthologue (29.6 and 28.1 kDa, respectively) that acts, singularly, to evoke the sea star predatory response. The orthologues (named "KEYSTONEin") are localized in the epidermis, extrapallial fluid, and organic shell coating (periostracum) of live, intact mussels. Thus, KEYSTONEin contacts chemosensory receptors on tube feet as sea stars crawl over rocky surfaces in search of prey. The complete nucleotide sequences reveal that KEYSTONEin shares 87% (M. californianus) or 98% (M. galloprovincialis) homology with a calcium-binding protein in the shell matrix of a closely related congener, M. edulis. All three molecules cluster tightly within the Complement Component 1 Domain Containing (C1qDC) protein family; each exhibits a large globular domain, low complexity region(s), coiled coil, and at least four of five histidine-aspartic acid tandem motifs. Collective results support the hypothesis that KEYSTONEin evolved ancestrally in immunological, and later, in biomineralization roles. More recently, the substance has become exploited by sea stars as a contact cue for prey recognition. As the first identified compound to evoke keystone predation, KEYSTONEin provides valuable sensory information, promotes biodiversity, and shapes community structure and function. Without this molecule, there would be no predation by sea stars on mussels.

A bio-process inspired synthesis of vaterite (CaCO3), directed by a rationally designed multifunctional protein, ChiCaSifi

A bio-process inspired synthesis of vaterite (CaCO₃) is achieved under the functions of a rationally designed multifunctional protein, ChiCaSifi.

Intracrystalline inclusions within single crystalline hosts: from biomineralization to bio-inspired crystal growth

A review of the inclusion of organic matter within single crystalline hosts: from biogenic minerals to bio-inspired nanohybrid single crystal composites.

Intrinsically disordered and pliable Starmaker-like protein from medaka (Oryzias latipes) controls the formation of calcium carbonate crystals

Fish otoliths, biominerals composed of calcium carbonate with a small amount of organic matrix, are involved in the functioning of the inner ear. Starmaker (Stm) from zebrafish (Danio rerio) was the first protein found to be capable of controlling the formation of otoliths. Recently, a gene was identified encoding the Starmaker-like (Stm-l) protein from medaka (Oryzias latipes), a putative homologue of Stm and human dentine sialophosphoprotein. Although there is no sequence similarity between Stm-l and Stm, Stm-l was suggested to be involved in the biomineralization of otoliths, as had been observed for Stm even before. The molecular properties and functioning of Stm-l as a putative regulatory protein in otolith formation have not been characterized yet. A comprehensive biochemical and biophysical analysis of recombinant Stm-l, along with in silico examinations, indicated that Stm-l exhibits properties of a coil-like intrinsically disordered protein. Stm-l possesses an elongated and pliable structure that is able to adopt a more ordered and rigid conformation under the influence of different factors. An in vitro assay of the biomineralization activity of Stm-l indicated that Stm-l affected the size, shape and number of calcium carbonate crystals. The functional significance of intrinsically disordered properties of Stm-l and the possible role of this protein in controlling the formation of calcium carbonate crystals is discussed.

Splice variants of perlucin from Haliotis laevigata modulate the crystallisation of CaCO3

Perlucin is one of the proteins of the organic matrix of nacre (mother of pearl) playing an important role in biomineralisation. This nacreous layer can be predominately found in the mollusc lineages and is most intensively studied as a compound of the shell of the marine Australian abalone Haliotis laevigata. A more detailed analysis of Perlucin will elucidate some of the still unknown processes in the complex interplay of the organic/inorganic compounds involved in the formation of nacre as a very interesting composite material not only from a life science-based point of view. Within this study we discovered three unknown Perlucin splice variants of the Australian abalone H. laevigata. The amplified cDNAs vary from 562 to 815 base pairs and the resulting translation products differ predominantly in the absence or presence of a varying number of a 10 mer peptide C-terminal repeat. The splice variants could further be confirmed by matrix-assisted laser desorption ionisation time of flight mass spectrometry (MALDI-ToF MS) analysis as endogenous Perlucin, purified from decalcified abalone shell. Interestingly, we observed that the different variants expressed as maltose-binding protein (MBP) fusion proteins in E. coli showed strong differences in their influence on precipitating CaCO₃ and that these differences might be due to a splice variant-specific formation of large protein aggregates influenced by the number of the 10 mer peptide repeats. Our results are evidence for a more complex situation with respect to Perlucin functional regulation by demonstrating that Perlucin splice variants modulate the crystallisation of calcium carbonate. The identification of differentially behaving Perlucin variants may open a completely new perspective for the field of nacre biomineralisation.

Mollusk shell structures and their formation mechanism

In nature, mollusk shells have a role in protecting the soft body of the mollusk from predators and from the external environment, and the shells consist mainly of calcium carbonate and small amounts of organic matrices. Organic matrices in mollusk shells are thought to play key roles in shell formation. However, enough information has not been accumulated so far. High toughness and stiffness have been focused on as being adaptable to the development of organic–inorganic hybrid materials. Because mollusks can produce elaborate microstructures containing organic matrices under ambient conditions, the investigation of shell formation is expected to lead to the development of new inorganic–organic hybrid materials for various applications. In this review paper, we summarize the structures of mollusk shells and their process of formation, together with the analysis of various organic matrices related to shell calcification.

The multiple effects of amino acids on the early stages of calcium carbonate crystallization

Proteins have found their way into many of Nature’s structures due to their structural stability, diversity in function and composition, and ability to be regulated as well as be regulators themselves. In this study, we investigate the constitutive amino acids that make up some of these proteins which are involved in CaCO3 mineralization – either in nucleation, crystal growth, or inhibition processes. By assaying all 20 amino acids with vapor diffusion and in situ potentiometric titration, we have found specific amino acids having multiple effects on the early stages of CaCO3 crystallization. These same amino acids have been independently implicated as constituents in liquid-like precursors that form mineralized tissues, processes believed to be key effects of biomineralization proteins in several biological model systems.

Prismin: a new matrix protein family in the Japanese pearl oyster (Pinctada fucata) involved in prismatic layer formation

The hard tissue of the Japanese pearl oyster, Pinctada fucata, consists of two layers, the outer prismatic layer, bearing calcite, and the inner nacreous layer, bearing aragonite. An EDTA-insoluble fraction of the prismatic layer of P. fucata was extracted with urea. In-vitro crystallization experiments showed that this urea-soluble fraction contained the factor(s) that promoted the growth of calcite crystals. We purified a protein from this fraction and deduced the internal amino acid sequences EYDFDRPDPYDP and EYDFERPD. We performed 3' RACE using primer DPPF1, encoding EYDFDRPDPYDP, and an oligo-dT adapter primer and amplified a fragment of approximately 300 bp. We screened cDNA libraries using the 300 bp fragment and obtained two clones that we named prismin 1 and 2. Both cDNAs encode proteins of 51 amino acids. Homology searches revealed 91% amino acid identity between prismin 1 and 2. The synthetic peptide DFDRPDPYDPYDRFD, corresponding to the carboxy terminal region of prismin 1, has calcite growing activity and calcium binding capability, showing that the carboxy-terminal region is a functional domain. Prismin 1 is expressed strongly in the outer edge and in the inner part of the mantle tissue. However, immunoblot analysis revealed that prismin protein exists only in the prismatic layer, not in the nacreous layer, despite the presence of the mRNA. Therefore, we conclude that prismin is a novel prismatic layer-specific calcite growth factor.

Molecular specifications of a mineral modulation sequence derived from the aragonite-promoting protein n16

In the nacre layer of the mollusk, proteins play an important role in regulating the morphology and lattice structure of calcium carbonate minerals. However, this process remains elusive due to the fact that we do not understand how protein sequences control the structure and morphology of biominerals. To take us a step further in this direction, we report the molecular structure of a 30 AA N-terminal mineral interactive sequence (n16N) of the aragonite-promoting protein, n16, and contrast these findings to those previously reported for two "calcite-blocker" nacre-associated sequences, AP7N and AP24N. We find that n16N is conformationally labile and adopts a random-coil conformation that possesses short, dispersed extended beta-strand segments that are located at the A1-Y2, K5-Y9, Y11-I14, and D21-N25 sequence blocks. Like AP7N and AP24N, Ca(II) ion interactions with n16N alter chain dynamics and local structure, and n16N is adsorbed onto calcite crystals and cannot easily be displaced via differential washing techniques. Furthermore, all three sequences have planar surface regions that could serve as putative sites for mineral interactions or ion cluster formation. However, what sets n16N apart from AP7N and AP24N are different folding propensities as well as unique molecular surface features and amino acid composition. n16N has a more condensed structure that, in the presence of TFE, folds into a beta-strand. This contrasts with the more open structures of AP7N and AP24N that are induced by TFE to fold into alpha-helices. Mapping of the n16N molecular surface reveals significant cationic regions and diffuse anionic charge, which contrasts with the small anionic "pocket" regions of AP7N/AP24N. Finally, n16N has 50% fewer sites for mineral surface- or ion cluster-associated water interactions compared to AP7N and AP24N. Overall, the structure of n16N is "tuned" to a different function within the in vitro mineralization scheme. The different features found in AP7N, AP24N, and n16N could be exploited for engineering polypeptides that recognize and bind to different surface features of inorganic crystalline solids.

Probing the organic-mineral interface at the molecular level in model biominerals

It is widely known that macromolecules, such as proteins, can control the nucleation and growth of inorganic solids in biomineralizing organisms. However, what is not known are the complementary molecular interactions, organization, and rearrangements that occur when proteins interact with inorganic solids during the formation of biominerals. The organic-mineral interface (OMI) is expected to be the site for these phenomena, and is therefore extraordinarily interesting to investigate. In this report, we employ X-ray absorption near edge (XANES) spectromicroscopy to investigate the electronic structure of both calcium carbonate mineral crystals and polypeptides, and detect changing bonds at the OMI during crystal growth in the presence of polypeptides. We acquired XANES spectra from calcium carbonate crystals grown in the presence of three mollusk nacre-associated polypeptides (AP7N, AP24N, n16N) and in the presence of a sea urchin spicule matrix protein, LSM34. All these model biominerals gave similar results, including the disruption of CO bonds in calcite and enhancement of the peaks associated with C-H bonds and C-O bonds in peptides, indicating ordering of the amino acid side chains in the mineral-associated polypeptides and carboxylate binding. This is the first evidence of the mutual effect of calcite on peptide chain and peptide chain on calcite during biomineralization. We also show that these changes do not occur when Asp and Glu are replaced in the n16N sequence with Asn and Gln, respectively, demonstrating that carboxyl groups in Asp and Glu do participate in polypeptide-mineral molecular associations.

Identification and structural characterization of an unusual RING-like sequence within an extracellular biomineralization protein, AP7

The RING or Really Interesting New Gene represents a family of eukaryotic sequences that bind Zn (II) ions and participate in intracellular processes involving protein-protein interaction. Although found in over 400 different proteins, very little is known regarding the structure-function properties of these domains because of the aggregation problems associated with RING sequences. To augment this data set, we report an unusual 36 AA C-terminal sequence of an extracellular matrix mollusk shell protein, AP7, that exhibits partial homology to the RING family. This Cys, His-containing sequence, termed AP7C, binds Zn (II) and other multivalent ions, but does not utilize a tetracoordinate complexation scheme for binding such as that found in Zn (II) finger polypeptides. Moreover, unlike Zn (II) finger and RING domains, this 36 AA can fold into a relatively stable structure in the absence of Zn (II). This folded structure consists of three short helical segments (A, B, and C), with segments A and B separated by a 4 AA type I beta-turn region and segments B and C separated by a 7 AA loop-like region. Interestingly, the putative RING-like region, -RRPFHECALCYSI-, experiences slow conformational exchange between two structural states in solution, most likely in response to imido ring interconversion at P8 and P21. Poisson-Boltzmann solvation calculations reveal that the AP7C molecular surface possesses a cationic region near its N-terminus, which lies adjacent to the 30 AA mineral modification domain in the AP7 protein. Given that the AP7C sequence does not influence mineralization, it is probable that this cationic pseudo-RING region is utilized by the AP7 protein for other tasks such as protein-protein interaction within the mollusk shell matrix.

Structural properties of an artificial protein that regulates the nucleation of inorganic and organic crystals

Technological advances have facilitated the generation of artificial proteins that possess the capabilities of recognizing and binding to inorganic solids and/or controlling nucleation processes that form inorganic solids. However, very little is known regarding the structure of these interesting polypeptides and how their structure contributes to functionality. To address this deficiency, we report structural investigations of an artificial protein, p288, that self-assembles and controls the nucleation of simple salts and organic compounds into dendrite-like crystals. Under aqueous conditions at low pH and in the presence of high salt, p288 is conformationally labile and exists primarily as a random coil conformer in equilibrium with other undefined secondary structures, including polyproline type II and beta turn. We note that p288 can fold into either a partial beta strand (at neutral pH) or a predominantly alpha helical (in the presence of TFE) conformation. Solid-state 13C-15N NMR experiments also reveal that p288 in the lyophilized, hydrated state possesses some degree of nonrandom coil structure. These results indicate that p288 is conformationally labile but can undergo conformational transitions to a more stable structure when water solvent loss/displacement occurs and protein concentrations increase. We believe that conformational instability and the ability to adopt different structures as a function of different environmental conditions represent important molecular features that impact p288 supramolecular assembly and crystal nucleation processes.

Structural Features That Distinguish Kinetically Distinct Biomineralization Polypeptides

AP7 and AP24 are mollusk shell proteins which are responsible for aragonite polymorph formation and stabilization within the nacre layer of the Pacific red abalone, Haliotis rufescens. It is known that the 30-AA N-terminal mineral modification domains of both proteins (AP7N, AP24N) possess identical multifunctional mineralization capabilities within in vitro assays but differ in terms of rate kinetics, with AP24N > AP7N. In this report, we identify previously unreported molecular features of AP24N and contrast the lowest energy polypeptide backbone structures of AP24N (planar configuration) with that of AP7N ("bent paper clip" configuration) using NMR data and simulated annealing molecular dynamics structure refinement. Like AP7N, we find that AP24N possesses an unfolded conformation, can sequester Ca(II) and other multivalent metal ions, can adsorb onto or within calcite crystals, and possesses anionic and cationic electrostatic "pocket" regions on its molecular surfaces. However, AP24N has some unique features: greater conformational responsiveness to Ca(II), the tendency to form a more planar backbone configuration, and longer anionic and hydrogen-bonding donor/acceptor sequence blocks. We conclude that the presence of unfolded polypeptide conformation, electrostatic surface pockets, and interactive sequence clustering endow both AP7N and AP24N with similar features that lead to comparable effects on crystal morphology and nucleation. However, AP24N possesses longer anionic and hydrogen-bonding sequence clusters and exhibits a tendency to adopt a more planar backbone configuration than AP7N does. We believe that these features facilitate peptide-mineral, peptide-ion, or water cluster interactions, thereby enhancing the mineralization kinetics of AP24N over AP7N.

Molluscan shell proteins: primary structure, origin, and evolution

In the last few years, the field of molluscan biomineralization has known a tremendous mutation, regarding fundamental concepts on biomineralization regulation as well as regarding the methods of investigation. The most recent advances deal more particularly with the structure of shell biominerals at nanoscale and the identification of an increasing number of shell matrix protein components. Although the matrix is quantitatively a minor constituent in the shell of mollusks (less than 5% w/w), it is, however, the major component that controls different aspects of the shell formation processes: synthesis of transient amorphous minerals and evolution to crystalline phases, choice of the calcium carbonate polymorph (calcite vs aragonite), organization of crystallites in complex shell textures (microstructures). Until recently, the classical paradigm in molluscan shell biomineralization was to consider that the control of shell synthesis was performed primarily by two antagonistic mechanisms: crystal nucleation and growth inhibition. New concepts and emerging models try now to translate a more complex reality, which is remarkably illustrated by the wide variety of shell proteins, characterized since the mid-1990s, and described in this chapter. These proteins cover a broad spectrum of pI, from very acidic to very basic. The primary structure of a number of them is composed of different modules, suggesting that these proteins are multifunctional. Some of them exhibit enzymatic activities. Others may be involved in cell signaling. The oldness of shell proteins is discussed, in relation with the Cambrian appearance of the mollusks as a mineralizing phylum and with the Phanerozoic evolution of this group. Nowadays, the extracellular calcifying shell matrix appears as a whole integrated system, which regulates protein-mineral and protein-protein interactions as well as feedback interactions between the biominerals and the calcifying epithelium that synthesized them. Consequently, the molluscan shell matrix may be a source of bioactive molecules that would offer interesting perspectives in biomaterials and biomedical fields.

Matrix proteins in the outer shells of molluscs

The shells of molluscs are composed mainly of calcium carbonate crystals, with small amounts of matrix proteins. For more than 50 years, they have attracted attention for their unique mechanical and biological properties. Only recently, however, have researchers begun to realize that it is the matrix proteins that control the formation of calcium carbonate crystals and play key roles in their extraordinary properties, despite the fact that matrix proteins comprise less than 5% of the shell weight. This article reviews the matrix proteins identified to date from the shells of molluscs, their structural characteristics, and their roles in shell formation. Some suggestions are given for further investigation based on the summary and analysis.

Probing the conformational features of a phage display polypeptide sequence directed against single-walled carbon nanohorn surfaces

Single-walled carbon nanohorns (SWNHs) are interesting carbon nanostructures that have applications to science and technology. Using M13 phage display technology, polypeptides directed again SWNHs surfaces have been created for a number of nanotechnology and pharmaceutical purposes, yet the molecular mechanism of polypeptide sequence interaction and binding to SWNHs surfaces is not known. Recently, we identified a linear 12-AA M13 phage pIII sequence, NH-12-5-2 (DYFSSPYYEQLF), that binds with high affinity to SWNHs surfaces. To probe the structure of this pIII tail polypeptide further, we investigated the conformation of a model peptide representing the 12 AA NH-12-5-2 sequence. At neutral pH, the NH-12-5-2 model polypeptide is conformationally labile and exhibits two-state conformational exchange involving the D1-S5 N-terminal segment. Simultaneous with this conformational exchange process is the observation that the P6 residue exhibits imido ring conformational variation. In the presence of the structure-stabilizing solvent, TFE, or at pH 2.5, both the exchange process and Pro ring motion phenomena disappear, indicating that the structure of this peptide sequence can be stabilized by extrinsic factors. Interestingly, we observe NMR parameters (ROEs, (3)J coupling constants) for NH-12-5-2 in 90% v/v TFE that are consistent with the presence of a partial helical structure, similar to what was observed at low pH in our earlier CD experiments. We conclude that the NH-12-5-2 model polypeptide sequence possesses an inherent conformational instability that involves the D1-S5 sequence segment and the P6 residue but that this instability can be offset by extrinsic factors (e.g., charge neutralization, imido ring interconversion, and hydrophobic-hydrophobic interactions). These nonbonding interactions may play a role in the recognition and binding of this phage sequence region to SWNHs surfaces.

Molecular characterization of the 30-AA N-terminal mineral interaction domain of the biomineralization protein AP7

The AP7 protein is one of several mollusk shell proteins which are responsible for aragonite polymorph formation and stabilization within the nacre layer of the Pacific red abalone, H. rufescens. Previously, we demonstrated that the 30-AA N-terminal domain of AP7, denoted as AP7-1, exists as an unfolded sequence and possesses the capability of inhibiting calcium carbonate crystal growth in vitro via growth step frustration or interruption. However, very little is known with regard to the interactive capabilities of this sequence with Ca(II) and with calcium carbonates. Using multidisciplinary techniques, we determine that the AP7-1 polypeptide interacts with Ca(II) ions at the -DD- sequence clusters, yet retains its unfolded, conformationally labile structure in the presence of Ca(II) ions. Further, NMR experiments reveal that the extended structured sequence blocks, -GNGM-, -SVRTQG-, and -ISYL, exhibit motional, chemical exchange, and/or backbone geometry perturbations in response to Ca(II) interactions with AP7-1. Solid-state NMR magic angle spinning studies verify that during the course of in vitro calcium carbonate crystal growth, AP7-1 becomes bound to calcite fragments and cannot be entirely displaced from the mineral fragments using competitive Ca(II) washing. Finally, using a scrambled sequence version of the AP7-1 polypeptide, we observe that sequence scrambling does not adversely affect the crystal growth inhibitory activity of AP7-1, suggesting that the amino acid composition of AP7-1 may be more critical to growth step inhibition than the linear ordering of amino acids.