Purification and characterization of perlucin and perlustrin, two new proteins from the shell of the mollusc Haliotis laevigata

Transcriptome and proteome analysis of Pinctada margaritifera calcifying mantle and shell: focus on biomineralization

BACKGROUND

The shell of the pearl-producing bivalve Pinctada margaritifera is composed of an organic cell-free matrix that plays a key role in the dynamic process of biologically-controlled biomineralization. In order to increase genomic resources and identify shell matrix proteins implicated in biomineralization in P. margaritifera, high-throughput Expressed Sequence Tag (EST) pyrosequencing was undertaken on the calcifying mantle, combined with a proteomic analysis of the shell.

RESULTS

We report the functional analysis of 276 738 sequences, leading to the constitution of an unprecedented catalog of 82 P. margaritifera biomineralization-related mantle protein sequences. Components of the current "chitin-silk fibroin gel-acidic macromolecule" model of biomineralization processes were found, in particular a homolog of a biomineralization protein (Pif-177) recently discovered in P. fucata. Among these sequences, we could show the localization of two other biomineralization protein transcripts, pmarg-aspein and pmarg-pearlin, in two distinct areas of the outer mantle epithelium, suggesting their implication in calcite and aragonite formation. Finally, by combining the EST approach with a proteomic mass spectrometry analysis of proteins isolated from the P. margaritifera shell organic matrix, we demonstrated the presence of 30 sequences containing almost all of the shell proteins that have been previously described from shell matrix protein analyses of the Pinctada genus. The integration of these two methods allowed the global composition of biomineralizing tissue and calcified structures to be examined in tandem for the first time.

CONCLUSIONS

This EST study made on the calcifying tissue of P. margaritifera is the first description of pyrosequencing on a pearl-producing bivalve species. Our results provide direct evidence that our EST data set covers most of the diversity of the matrix protein of P. margaritifera shell, but also that the mantle transcripts encode proteins present in P. margaritifera shell, hence demonstrating their implication in shell formation. Combining transcriptomic and proteomic approaches is therefore a powerful way to identify proteins involved in biomineralization. Data generated in this study supply the most comprehensive list of biomineralization-related sequences presently available among protostomian species, and represent a major breakthrough in the field of molluskan biomineralization.

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.

Promoter complexity and tissue-specific expression of stress response components in Mytilus galloprovincialis, a sessile marine invertebrate species

The mechanisms of stress tolerance in sessile animals, such as molluscs, can offer fundamental insights into the adaptation of organisms for a wide range of environmental challenges. One of the best studied processes at the molecular level relevant to stress tolerance is the heat shock response in the genus Mytilus. We focus on the upstream region of Mytilus galloprovincialis Hsp90 genes and their structural and functional associations, using comparative genomics and network inference. Sequence comparison of this region provides novel evidence that the transcription of Hsp90 is regulated via a dense region of transcription factor binding sites, also containing a region with similarity to the Gamera family of LINE-like repetitive sequences and a genus-specific element of unknown function. Furthermore, we infer a set of gene networks from tissue-specific expression data, and specifically extract an Hsp class-associated network, with 174 genes and 2,226 associations, exhibiting a complex pattern of expression across multiple tissue types. Our results (i) suggest that the heat shock response in the genus Mytilus is regulated by an unexpectedly complex upstream region, and (ii) provide new directions for the use of the heat shock process as a biosensor system for environmental monitoring.

The structure-function relationship of MSI7, a matrix protein from pearl oyster Pinctada fucata

We previously identified a matrix protein, MSI7, from pearl oyster Pinctada fucata. According to the structural analysis, the DGD site in the N-terminal of MSI7 is crucial for its role in the shell formation. In this study, we expressed a series of recombinant MSI7 proteins, including the wild-type and several mutants directed at the DGD site, using an Escherichia coli expression system to reveal the structure-function relationship of MSI7. Furthermore, in vitro crystallization, crystallization speed assay, and circular dichroism spectrometry were carried out. Results indicated that wild-type MSI7 could induce the nucleation of aragonite and inhibit the crystallization of calcite. However, none of the mutants could induce the nucleation of aragonite, but all of them could inhibit the crystallization of calcite to some extent. And all the proteins accelerated the crystallization process. Taken together, the results indicated that MSI7 could contribute to aragonite crystallization by inducing the nucleation of aragonite and inhibiting the crystallization of calcite, which agrees with our prediction about its role in the nacreous layer formation of the shell. The DGD site was critical for the induction of the nucleation of aragonite.

[Progresses on immune-related genes and proteins of abalones]

Abalones, belonging to one of the largest marine gastropod mollusks, are economically important seafood in aquaculture worldwide. In recent years, bacterial epidemic infection has been reported in China and other countries, and mass mortality in abalones causes significant economic losses. Immune-related genes and proteins of abalones are seldom reported. However, these functional molecules may play a key role in resisting diseases and maintaining healthy status and are pivotal for studying immunological mechanisms. Here we summarized the advanced research and progresses in abalone immune-related genes and proteins with the purpose of facilitating future study of these target molecules involved in immunological mechanisms.

Visualization of shell matrix proteins in hemocytes and tissues of the Eastern oyster, Crassostrea virginica

The tissues of the oyster were examined for the presence of shell matrix proteins (SMPs) using a combination of Western, proteomic, and epi-fluorescent microscopy techniques. SMP, including 48 and 55 kDa phosphoproteins, was detected in the epithelial cells of mantle, gill, heart, and adductor muscle and linings of arteries and veins. The 48 kDa SMP circulates continuously within the hemolymph, and is present in the immune system hemocytes. It appears to be secreted from hemocytes on induction of shell repair. We suggest that the 48 and 55 kDa proteins are multifunctional and bridge the process of soft tissue repair and shell formation by mediating cellular activities during immune response as well as interacting with the mineral phase during deposition.

The inner-shell film: an immediate structure participating in pearl oyster shell formation

In mollusks, the inner shell film is located in the shell-mantle zone and it is important in shell formation. In this study, we found that the film was composed of two individual films under certain states and some columnar structures were observed between the two individual films. The inner shell film was separated with the process of ethylenediaminetetraacetic acid (EDTA) treatment and the film proteins were extracted. Amino acid analysis showed that the film proteins may consist of shell framework proteins. The calcite crystallization experiment showed that the film proteins could inhibit the growth of calcite, while the CaCO(3) precipitation experiment showed that the film proteins could accelerate the rate of CaCO(3) precipitation. All these results suggested that the film plays an important role in shell formation. It may facilitate the aragonite formation by inhibiting the growth of calcite and accelerate the shell growth by promoting the precipitation of CaCO(3) crystals.

Low molecular weight molecules of oyster nacre induce mineralization of the MC3T3-E1 cells

The nacre layer from the pearl oyster shell is considered as a promising osteoinductive biomaterial. Nacre contains one or more signal molecules capable of stimulating bone formation. The identity and the mode of action of these molecules on the osteoblast differentiation were analyzed. Water-soluble molecules from nacre were fractionated according to dialysis, solvent extraction, and reversed-phase HPLC. The activity of a fraction composed of low molecular weight molecules in the mineralization of the MC3T3-E1 extracellular matrix was investigated. Mineralization of the preosteoblast cells was monitored according to alizarin red staining, Raman spectroscopy, scanning electron microscopy, and quantitative RT-PCR. Molecules isolated from nacre, ranging from 50 to 235 Da, induced a red alizarin staining of the preosteoblasts extracellular matrix after 16 days of culture. Raman spectroscopy demonstrated the presence of hydroxyapatite (HA) in samples treated with these molecules. Scanning electron microscopy pictures showed at the surface of the treated cells the occurrence of clusters of spherical particles resembling to HA. The treatment of cells with nacre molecules accelerated expression of collagen I and increased the mRNA expression of Runx2 and osteopontin. This study indicated that the nacre molecules efficient in bone cell differentiation are certainly different from proteins, and could be useful for in vivo bone repair.

Recombinant perlucin nucleates the growth of calcium carbonate crystals: molecular cloning and characterization of perlucin from disk abalone, Haliotis discus discus

Perlucin is well known as an important functional protein regulating pearl formation and shell biomineralization. In this study, we cloned the perlucin gene from the abalone Haliotis discus discus cDNA library. The full-length cDNA of the abalone H. discus discus perlucin gene consisted of 1038 bp nucleotides, encoding a putative signal peptide of 22 amino acids and a mature protein of 129 amino acids, which shared 55% identity with the homologous protein in greenlip abalone. The mature protein coding sequence was inserted into pMal-c2X expression vector and it expressed the recombinant protein in E. coli (Rosetta-gammi DE3). The maltose binding protein (MBP) fusion perlucin successfully promoted calcium carbonate precipitation and directed calcite crystal morphological modification. The successful expression of active recombinant perlucin suggested that recombinant perlucin gene transfer has the capability by color modification to improve the pearl's value. In the view of molecular structure, perlucin was a typical C-type lectin, which contained one highly conserved carbohydrate recognition domain. Reverse transcription polymerase chain reaction (RT-PCR) results showed that perlucin gene was expressed not only in the mantle, but also in the gill and digestive tract. Further characterization of perlucin in abalone non-self recognition and disease resistance is promising and anticipated.

N40, a novel nonacidic matrix protein from pearl oyster nacre, facilitates nucleation of aragonite in vitro

A novel nonacidic matrix protein from pearl oyster nacre has been purified by cation-exchange chromatography. It was designated N40 for the nacreous protein of approximately 40 kDa. On the basis of the extraction method (with Tris-buffered Milli-Q water) and amino acid compositions (Gly- and Ala-rich), N40 was inferred to be a conventional "insoluble matrix protein". Crystallization experiments showed that N40 could facilitate the nucleation of aragonite drastically. So far, among the macromolecules that have been purified from the shell, N40 is an exclusive protein that can nucleate aragonite by itself, without the need for adsorption to a substrate. Thus, the present study has proposed the possibility that the nonacidic shell protein (maybe a conventional "insoluble framework protein") can also directly participate in aragonite nucleation and even act as a nucleation site. It is a valuable supplement to the classic biomineralization theory, in which the soluble acidic proteins of the shell are generally believed to function as a nucleation site.

The structure-function relationship analysis of Prismalin-14 from the prismatic layer of the Japanese pearl oyster, Pinctada fucata

The mollusk shell is a hard tissue consisting of calcium carbonate and organic matrices. The organic matrices are considered to play important roles in shell formation. We have previously identified a prismatic layer-specific protein named Prismalin-14, which consists of 105 amino acid residues and includes four structurally characteristic regions; a repeated sequence of Pro-Ile-Tyr-Arg, a Gly/Tyr-rich region and N- and C-terminal Asp-rich regions. Prismalin-14 showed an inhibitory activity on calcium carbonate precipitation and a calcium-binding ability in vitro. In this study, we prepared some molecular species of recombinant proteins including Prismalin-14 and its truncated proteins in an Escherichia coli expression system to reveal a structure-function relationship of Prismalin-14. The results showed that the Gly/Tyr-rich region was responsible for chitin binding and was identified as a novel chitin-binding sequence. On the other hand, both N- and C-terminal Asp-rich regions are related to inhibitory activity on calcium carbonate precipitation in vitro. Immunohistological observation revealed that Prismalin-14 was localized at the acid-insoluble organic framework including chitin. All these results strongly suggest that Prismalin-14 is a framework protein that mediates chitin and calcium carbonate crystals by using its acidic and chitin-binding regions.

A novel nacre protein N19 in the pearl oyster Pinctada fucata

A novel 19kDa protein, which was named N19, was isolated from the nacreous layer of the pearl oyster Pinctada fucata. N19 is one of predominant proteins found in the water-insoluble fraction of the nacreous layer. MALDI-TOF/TOF analysis indicated that the three trypsin-digested peptides (791.45, 824.42, and 1118.65m/z) corresponded to the amino acid sequences predicted from a cDNA isolated from a mantle cDNA library of P. fucata. Northern blot analysis revealed that the N19 mRNA was a little more abundant in the pallial region than the edge region, in the mantle. In CaCO(3) precipitation assay, the recombinant N19 protein inhibited the crystallization of CaCO(3). These results indicate that N19 is localized in the nacre and plays a negative regulatory role in calcification in the pearl oyster.

A Synthesis Approach to Understanding Repeated Peptides Conserved in Mineralization Proteins

We created artificial proteins that contained repeats of a short peptide motif, Asn-Gly-Asx. In nature this motif is repeated within shell proteins as an idiosyncratic domain, while in vitro it has been shown to suppress calcification. The motif was embedded within peptide sequences that did or did not have the ability to form secondary structures, which provided the motif with a variety of physicochemical properties. Although a short synthetic peptide containing the motif did not inhibit calcification in vitro, some of the artificial proteins carrying repeats of the motif did show robust suppression of calcification. Artificial proteins lacking the motif did not exhibit suppressive activity. Likewise, one construct containing multiple repeats of the motifs also did not exert an inhibitory effect on calcification. Apparently, carrying the Asn-Gly-Asx motif is not, by itself, sufficient for expression of its cryptic activity; instead, certain physicochemical properties of the polypeptides mediate its manifestation. We anticipate that syntheses using "motif programming", such as the one described here, will shed light on the origin of repetitive sequences as well as on the evolution of biomineralization proteins.

Neuroparsins, a family of conserved arthropod neuropeptides

Different neuroparsin variants were initially identified as anti-gonadotropic peptides from the pars intercerebralis-corpora cardiaca complex of the migratory locust, Locusta migratoria, and further studies revealed the pleiotropic activities of these peptides. Subsequently, additional neuroparsin-like peptides were discovered from other arthropod species. Studies in mosquitoes and locusts suggest that members of this conserved peptide family are involved in the regulation of insect reproduction and can even serve as molecular markers of the fascinating biological process of locust phase transition. Sequence analysis and multiple alignments revealed pronounced sequence similarities between arthropod neuroparsins and the N-terminal, growth factor binding region of vertebrate and mollusc insulin-like growth factor binding proteins (IGFBP). This observation led to the hypothesis that neuroparsins might interact with endogenous insulin-related peptides. The present paper gives an overview of several neuroparsin family members that have hitherto been described in insects, as well as of a number of newly identified neuroparsin precursors from other species.

Heterogeneity of proteinase inhibitors in the water-soluble organic matrix from the oyster nacre

We extracted proteinase inhibitors from the nacre of the oyster Pinctada margaritifera with water. Mixing the nacre powder with water for 20 h led to a water-soluble fraction [0.24% (wt/wt) of nacre]. After dialysis of the water-soluble matrix through 6- to 8-kDa and 0.5-kDa membranes, the proteinase inhibitors were divided into low and high molecular weight fractions that contained inhibitors of papain, bovine cathepsin B, and human cathepsin L. We studied the heterogeneity of the inhibitors after separating the low molecular weight fraction according to charge and hydrophobicity. After multistep purification, mass spectrometry analysis revealed that a potent inhibitory fraction contained several molecules. This observation demonstrates the difficulties encountered in attempting to isolate individual metabolites from the complex mixture of molecules present in nacre matrix. Interestingly, the low molecular weight fraction contained specific inhibitors that could discern between cathepsin B and cathepsin L. The nacre organic inhibitors were active against several cysteine proteinases, yet they were more specific in relation to serine proteinases, because only proteinase K was inhibited. These results demonstrate, for the first time, the presence of active proteinase inhibitors in the mollusc shell, and it is possible that these inhibitors may play a role in either protection of proteins involved in shell formation or in defense against parasites, or both.

Characterization and in vitro mineralization function of a soluble protein complex P60 from the nacre of Pinctada fucata

A soluble protein complex P60 from the powdered nacre of Pinctada fucata was extracted and partially characterized. The biological activity of the P60 on pre-osteoblast cell line MC3T3-E1 and bone marrow stroma cells (MSCs) was investigated. The P60 protein from the decalcified powered nacre was solubilized with acetic acid and then purified by liquid chromatography. The P60 protein was a protein complex composed of several subunits with disulfide bridges. The known protein nacrein, and its two derivatives, N28 and N35, were included in the P60 protein complex. The most abundant amino acids in the P60 that account for 68.3% of the total residues are glycine (32.1%), aspartic acid (17.4%), alanine (13.6%), and glutamic acid (5.2%). The in vitro study of the crystallization showed that this protein complex could control the formation and size of calcium carbonate. The assay of biological activity of the P60 protein complex on the pre-osteoblast cell line MC3T3-E1 and MSCs suggested that the P60 could stimulate the formation of mineralized nodules.

Perlinhibin, a cysteine-, histidine-, and arginine-rich miniprotein from abalone (Haliotis laevigata) nacre, inhibits in vitro calcium carbonate crystallization

We have isolated a 4.785 Da protein from the nacreous layer of the sea snail Haliotis laevigata (greenlip abalone) shell after demineralization with acetic acid. The sequence of 41 amino acids was determined by Edman degradation supported by mass spectrometry. The most abundant amino acids were cysteine (19.5%), histidine (17%), and arginine (14.6%). The positively charged amino acids were almost counterbalanced by negatively charged ones resulting in a calculated isoelectric point of 7.86. Atomic-force microscopy studies of the interaction of the protein with calcite surfaces in supersaturated calcium carbonate solution or calcium chloride solution showed that the protein bound specifically to calcite steps, inhibiting further crystal growth at these sites in carbonate solution and preventing crystal dissolution when carbonate was substituted with chloride. Therefore this protein was named perlinhibin. X-ray diffraction investigation of the crystal after atomic-force microscopy growth experiments showed that the formation of aragonite was induced on the calcite substrate around holes caused by perlinhibin crystal-growth inhibition. The strong interaction of the protein with calcium carbonate was also shown by vapor diffusion crystallization. In the presence of the protein, the crystal surfaces were covered with holes due to protein binding and local inhibition of crystal growth. In addition to perlinhibin, we isolated and sequenced a perlinhibin-related protein, indicating that perlinhibin may be a member of a family of closely related proteins.

The shell matrix of the freshwater mussel Unio pictorum (Paleoheterodonta, Unionoida). Involvement of acidic polysaccharides from glycoproteins in nacre mineralization

Among molluscs, the shell biomineralization process is controlled by a set of extracellular macromolecular components secreted by the calcifying mantle. In spite of several studies, these components are mainly known in bivalves from only few members of pteriomorph groups. In the present case, we investigated the biochemical properties of the aragonitic shell of the freshwater bivalve Unio pictorum (Paleoheterodonta, Unionoida). Analysis of the amino acid composition reveals a high amount of glycine, aspartate and alanine in the acid-soluble extract, whereas the acid-insoluble one is rich in alanine and glycine. Monosaccharidic analysis indicates that the insoluble matrix comprises a high amount of glucosamine. Furthermore, a high ratio of the carbohydrates of the soluble matrix is sulfated. Electrophoretic analysis of the acid-soluble matrix revealed discrete bands. Stains-All, Alcian Blue, periodic acid/Schiff and autoradiography with (45)Ca after electrophoretic separation revealed three major polyanionic calcium-binding glycoproteins, which exhibit an apparent molecular mass of 95, 50 and 29 kDa, respectively. Two-dimensional gel electrophoresis shows that these bands, provisionally named P95, P50 and P29, are composed of numerous isoforms, the majority of which have acidic isoelectric points. Chemical deglycosylation of the matrix with trifluoromethanesulfonic acid induces a drastic shift of both the apparent molecular mass and the isoelectric point of these matrix components. This treatment induces also a modification of the shape of CaCO(3) crystals grown in vitro and a loss of the calcium-binding ability of two of the main matrix proteins (P95 and P50). Our findings strongly suggest that post-translational modifications display important functions in mollusc shell calcification.

Identification and characterization of a biomineralization related gene PFMG1 highly expressed in the mantle of Pinctada fucata

To elucidate the mechanism of nacre biomineralization, the mantle of Pinctada fucata (P. fucata) from the South China Sea was used. Using the mantle cDNA library and the ESTs we have cloned through suppression subtractive hybridization (SSH), ten novel genes including PFMG1 were obtained through nested PCR. Bioinformative results showed that PFMG1 had a high homology (40%) with Onchocerca volvulus calcium-binding protein CBP-1 and had two EF-hand calcium-binding domains from the 81st to the 93rd amino acid and from the 98th to the 133rd amino acid in the deduced amino acid sequence. The results of multitissue RT-PCR and in situ hybridization demonstrated the high expression of PFMG1 in the mantle of P. fucata and confirmed the SSH method. The results of GST-PFMG1 on CaCO3 crystallization showed significant effects on nucleation and precipitation of CaCO3. PFMG1 was cloned into the pcDNA.3.1/myc-HisA vector and was subsequently transfected into MC3T3-E1 cells. RT-PCR revealed upregulation of the marker genes related to cell growth, differentiation, and mineralization, and BMP-2, osterix, and osteopontin were upregulated as a result. This research work suggests that PFMG1 plays an important role in the nacre biomineralization, and the SSH method can pave the way for the bulk cloning and characterization of new genes involved in biomineralization in P. fucata and may accelerate research on the mechanism of pearl formation.

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.

A rapidly evolving secretome builds and patterns a sea shell

Background

Instructions to fabricate mineralized structures with distinct nanoscale architectures, such as seashells and coral and vertebrate skeletons, are encoded in the genomes of a wide variety of animals. In mollusks, the mantle is responsible for the extracellular production of the shell, directing the ordered biomineralization of CaCO₃ and the deposition of architectural and color patterns. The evolutionary origins of the ability to synthesize calcified structures across various metazoan taxa remain obscure, with only a small number of protein families identified from molluskan shells. The recent sequencing of a wide range of metazoan genomes coupled with the analysis of gene expression in non-model animals has allowed us to investigate the evolution and process of biomineralization in gastropod mollusks.

Results

Here we show that over 25% of the genes expressed in the mantle of the vetigastropod Haliotis asinina encode secreted proteins, indicating that hundreds of proteins are likely to be contributing to shell fabrication and patterning. Almost 85% of the secretome encodes novel proteins; remarkably, only 19% of these have identifiable homologues in the full genome of the patellogastropod Lottia scutum. The spatial expression profiles of mantle genes that belong to the secretome is restricted to discrete mantle zones, with each zone responsible for the fabrication of one of the structural layers of the shell. Patterned expression of a subset of genes along the length of the mantle is indicative of roles in shell ornamentation. For example, Has-sometsuke maps precisely to pigmentation patterns in the shell, providing the first case of a gene product to be involved in molluskan shell pigmentation. We also describe the expression of two novel genes involved in nacre (mother of pearl) deposition.

Conclusion

The unexpected complexity and evolvability of this secretome and the modular design of the molluskan mantle enables diversification of shell strength and design, and as such must contribute to the variety of adaptive architectures and colors found in mollusk shells. The composition of this novel mantle-specific secretome suggests that there are significant molecular differences in the ways in which gastropods synthesize their shells.

A novel matrix protein p10 from the nacre of pearl oyster (Pinctada fucata) and its effects on both CaCO3 crystal formation and mineralogenic cells

A novel matrix protein, designated as p10 because of its apparent molecular mass of 10 kDa, was isolated from the nacreous layer of pearl oyster (Pinctada fucata) by reverse-phase high-performance liquid chromatography. In vitro crystallization experiments showed that p10 could accelerate the nucleation of calcium carbonate crystals and induce aragonite formation, suggesting that it might play a key role in nacre biomineralization. As nacre is known to contain osteogenic factors, two mineralogenic cell lines, MRC-5 fibroblasts and MC3T3-E1 preosteoblasts, were used to investigate the biological activity of p10. The results showed that p10 could increase alkaline phosphatase activity, an early marker of osteoblast differentiation, while the viability of MRC-5 and MC3T3-E1 remained unchanged after treatment of p10. Taken together, the findings led to identification of a novel matrix protein from the nacre of P. fucata that plays a role in both the mineral phase and in the differentiation of the cells involved in biomineralization.

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.

Perlwapin, an abalone nacre protein with three four-disulfide core (whey acidic protein) domains, inhibits the growth of calcium carbonate crystals

We have isolated a new protein from the nacreous layer of the shell of the sea snail Haliotis laevigata (abalone). Amino acid sequence analysis showed the protein to consist of 134 amino acids and to contain three sequence repeats of approximately 40 amino acids which were very similar to the well-known whey acidic protein domains of other proteins. The new protein was therefore named perlwapin. In addition to the major sequence, we identified several minor variants. Atomic force microscopy was used to explore the interaction of perlwapin with calcite crystals. Monomolecular layers of calcite crystals dissolve very slowly in deionized water and recrystallize in supersaturated calcium carbonate solution. When perlwapin was dissolved in the supersaturated calcium carbonate solution, growth of the crystal was inhibited immediately. Perlwapin molecules bound tightly to distinct step edges, preventing the crystal layers from growing. Using lower concentrations of perlwapin in a saturated calcium carbonate solution, we could distinguish native, active perlwapin molecules from denaturated ones. These observations showed that perlwapin can act as a growth inhibitor for calcium carbonate crystals in saturated calcium carbonate solution. The function of perlwapin in nacre growth may be to inhibit the growth of certain crystallographic planes in the mineral phase of the polymer/mineral composite nacre.

Shematrin: A family of glycine-rich structural proteins in the shell of the pearl oyster Pinctada fucata

Random sequencing of molecules from a cDNA library constructed from mantle mRNA of the pearl oyster Pinctada fucata was used to obtain information on organic matrix proteins in the shell. In the determined sequences, we identified 7 distinct cDNAs encoding similar glycine-rich domains. Complete sequence analysis of these cDNAs showed that the predicted sequences of the proteins, which we named shematrins, possessed similar domains comprising repeat sequences of two or more glycines, followed by a hydrophobic amino acid. In addition, in shematrin-1, -2 and -3, a repeat domain designated as XGnX (where X is a hydrophobic amino acid) was conserved. It is of further note that all the shematrin proteins have RKKKY, RRKKY or RRRKY as their C-terminal sequence. According to northern blot analysis, all shematrins are exclusively expressed in the mantle, and particularly in the edge region of the mantle; furthermore, peptide fragments similar to shematrin-1 and -2 were detected in the prismatic layer of shells by MALDI-TOF/TOF MS analysis. These findings suggest that many of shematrins are synthesized in the mantle edge and secreted into the prismatic layer of the shell, where the protein family is thought to provide a framework for calcification.

Molecular Evolution and Functionally Important Structures of Molluscan Dermatopontin: Implications for the Origins of Molluscan Shell Matrix Proteins

A major shell matrix protein originally obtained from a freshwater snail is a molluscan homologue of Dermatopontins, a group of Metazoan proteins also called TRAMP (tyrosine-rich acidic matrix protein). We sequenced and identified 14 molluscan homologues of Dermatopontin from eight snail species belonging to the order Basommatophora and Stylommatophora. The bassommatophoran Dermatopontins fell into three types, one is suggested to be a shell matrix protein and the others are proteins having more general functions based on gene expression analyses. N-glycosylation is inferred to be important for the function involved in shell calcification, because potential N-glycosylation sites were found exclusively in the Dermatopontins considered as shell matrix proteins. The stylommatophoran Dermatopontins fell into two types, also suggested to comprise a shell matrix protein and a protein having a more general function. Phylogenetic analyses using maximum likelihood and Bayesian methods revealed that gene duplication events occurred independently in both basommatophoran and stylommatophoran lineages. These results suggest that the dermatopontin genes were co-opted for molluscan calcification at least twice independently after the divergence of basommatophoran and stylommatophoran lineages, or more recently than we have expected.

The C-type lectin-like domain superfamily

The superfamily of proteins containing C-type lectin-like domains (CTLDs) is a large group of extracellular Metazoan proteins with diverse functions. The CTLD structure has a characteristic double-loop ('loop-in-a-loop') stabilized by two highly conserved disulfide bridges located at the bases of the loops, as well as a set of conserved hydrophobic and polar interactions. The second loop, called the long loop region, is structurally and evolutionarily flexible, and is involved in Ca2+-dependent carbohydrate binding and interaction with other ligands. This loop is completely absent in a subset of CTLDs, which we refer to as compact CTLDs; these include the Link/PTR domain and bacterial CTLDs. CTLD-containing proteins (CTLDcps) were originally classified into seven groups based on their overall domain structure. Analyses of the superfamily representation in several completely sequenced genomes have added 10 new groups to the classification, and shown that it is applicable only to vertebrate CTLDcps; despite the abundance of CTLDcps in the invertebrate genomes studied, the domain architectures of these proteins do not match those of the vertebrate groups. Ca2+-dependent carbohydrate binding is the most common CTLD function in vertebrates, and apparently the ancestral one, as suggested by the many humoral defense CTLDcps characterized in insects and other invertebrates. However, many CTLDs have evolved to specifically recognize protein, lipid and inorganic ligands, including the vertebrate clade-specific snake venoms, and fish antifreeze and bird egg-shell proteins. Recent studies highlight the functional versatility of this protein superfamily and the CTLD scaffold, and suggest further interesting discoveries have yet to be made.

Caspartin and Calprismin, Two Proteins of the Shell Calcitic Prisms of the Mediterranean Fan Mussel Pinna nobilis

We used the combination of preparative electrophoresis and immunological detection to isolate two new proteins from the shell calcitic prisms of Pinna nobilis, the Mediterranean fan mussel. The amino acid composition of these proteins was determined. Both proteins are soluble, intracrystalline, and acidic. The 38-kDa protein is glycosylated; the 17-kDa one is not. Ala, Asx, Thr, and Pro represent the dominant residues of the 38-kDa protein, named calprismin. An N-terminal sequence was obtained from calprismin. This sequence, which comprises a pattern of 4 cysteine residues, is not related to any known protein. The second protein, named caspartin, exhibits an unusual amino acid composition, since Asx constitutes by far the main amino acid residue. Preliminary sequencing surprisingly suggests that the first 75 N-terminal residues are all Asp. Caspartin self-aggregates spontaneously into multimers. In vitro tests show that it inhibits the precipitation of calcium carbonate. Furthermore, it strongly interferes with the growth of calcite crystals. A polyclonal antiserum raised against caspartin was used to localize this protein in the shell by immunogold. The immunolocalization demonstrates that caspartin is distributed within the prisms and makes a continuous film at the interface between the prisms and the surrounding insoluble sheets. Our finding emphasizes the prominent role of aspartic acid-rich proteins for the building of calcitic prisms among molluscs.

Changes in eggshell mechanical properties, crystallographic texture and in matrix proteins induced by moult in hens

The effect of moult on eggshell mechanical properties, on composition and concentrations of organic matrix components and on eggshell microstructure was investigated. The observed changes were studied to understand the role of organic matrix and eggshell microstructure in eggshell strength. Moult was induced by zinc oxide (20 g zinc/kg diet) in 53 ISA Brown laying hens at 78 weeks of age. No difference was observed for egg or eggshell weights after moult. In contrast, moult improved the shell breaking strength (28.09 vs 33.71 N). After moult, there was a decrease in the average size of calcite crystals composing the eggshell and in their heterogeneity, whereas crystal orientation remained basically the same. After moulting, the total protein concentration in eggshell increased slightly. The comparisons of SDS-PAGE profiles of the organic matrix constituents extracted before and after moulting showed changes in staining intensity of certain bands. After moult, bands associated with main proteins specific to eggshell formation (OC-116 and OC-17) showed higher staining intensity, while the intensity of the egg white proteins (ovotransferrin, ovalbumin and lysozyme) decreased. ELISA confirmed the decrease in ovotransferrin after moult. Its concentration was inversely correlated with breaking strength before moult. These observations suggest that changes in eggshell crystal size could be due to changes in organic matrix composition. These changes may provide a mechanism for the improvement in shell solidity after moulting.

Amorphous layer around aragonite platelets in nacre

We reveal that the aragonite CaCO3 platelets in nacre of Haliotis laevigata are covered with a continuous layer of disordered amorphous CaCO3 and that there is no protein interaction with this layer. This finding contradicts classical paradigms of biomineralization, e.g., an epitaxial match between the structural organic matrix and the formed mineral. This finding also highlights the role of physicochemical effects in morphogenesis, complementing the previously assumed total control by biomolecules and bioprocesses, with many implications in nanotechnology and materials science.

Structural Characterization of the Major Extrapallial Fluid Protein of the Mollusc Mytilus edulis: Implications for Function

The major protein component of the extrapallial fluid of the mollusc Mytilus edulis has been previously isolated and partially characterized. It was postulated to play a role in shell mineralization because of its intriguing property of Ca(2+)-binding-induced self-assembling. However, it also binds other divalent ions, including Cd(2+), Cu(2+), Mn(2+), and Mg(2+). Herein is the initial report on the characterization of the primary structure of the extrapallial (EP) protein by RT-PCR and cDNA sequencing methods and by de novo peptide sequencing with mass spectrometry. The EP protein is comprised of 213 amino acids postcleavage of a signal peptide of 23 amino acids. The protein is rich in His, Glu, and Asp residues. The site of N-glycosylation, "NHTE", at amino acid positions 54-57 and the intramolecular disulfide bond between Cys 139 and Cys 171 of the protein have been characterized also. Sequence comparisons reveal that the EP protein possesses little homology to any presently known matrix proteins previously isolated from mollusc shells but rather it highly resembles a heavy metal binding protein and a histidine-rich glycoprotein, both from the hemolymph of M. edulis. The predicted domain profile and amino acid composition suggest that its N-terminus may be involved in calcium binding. The abundance of histidine residues of the protein may account for its heavy metal binding properties. Thus, the EP protein perhaps has multiple functions, serving as a Ca(2+)-transport protein, a shell matrix protein, and a heavy metal detoxification protein.

Suppressive subtractive hybridization of and differences in gene expression content of calcifying and noncalcifying cultures of Emiliania huxleyi strain 1516

The marine coccolithophorid Emiliania huxleyi is a cosmopolitan alga intensely studied in relation to global carbon cycling, biogeochemistry, marine ecology, and biomineralization processes. The biomineralization capabilities of coccolithophorids have attracted the attention of scientists interested in exploiting this ability for the development of materials science and biomedical and biotechnological applications. Although it has been well documented that biomineralization in E. huxleyi is promoted by growth under phosphate-limited conditions, the genes and proteins that govern the processes of calcification and coccolithogenesis remain unknown. Suppressive subtractive hybridization (SSH) libraries were constructed from cultures grown in phosphate-limited and phosphate-replete media as tester and driver populations for reciprocal SSH procedures. Positive clones from each of the two libraries were randomly selected, and dot blotting was performed for the analysis of expression patterns. A total of 513 clones from the phosphate-replete library and 423 clones from the phosphate-limited library were sequenced, assembled, and compared to sequences in GenBank using BLASTX. Of the 103 differentially expressed gene fragments from the phosphate-replete library, 34% showed significant homology to other known proteins, while only 23% of the 65 differentially expressed gene fragments from the phosphate-limited library showed homology to other proteins. To further assess mRNA expression, real-time RT-PCR analysis was employed and expression profiles were generated over a 14-day time course for three clones from the phosphate-replete library and five clones from the phosphate-limited library. The fragments isolated provide the basis for future cloning of full-length genes and functional analysis.

Cloning and characterization of an mRNA encoding a novel G protein alpha-subunit abundant in mantle and gill of pearl oyster Pinctada fucata

Nacre formation is an ideal model to study biomineralization processes. Although much has been done about biomineralization mechanism of nacre, little is known as to how cellular signaling regulates this process. We are interested in whether G protein signaling plays a role in mineralization. Degenerate primers against conserved amino acid regions of G proteins were employed to amplify cDNA from the pearl oyster Pinctada fucata. As a result, the cDNA encoding a novel G(s)alpha (pfG(s)alpha) from the pearl oyster was isolated. The G(s)alpha cDNA encodes a polypeptide of 377 amino acid residues, which shares high similarity to the octopus (Octopus vulgaris) G(s)alpha. The well-conserved A, C, G (switch I), switch II functional domains and the carboxyl terminus that is a critical site for interaction with receptors are completely identical to those from other mollusks. However, pfG(s)alpha has a unique amino acid sequence, which encodes switch III and interaction sites of adenylyl cyclase respectively. In situ hybridization and Northern blotting analysis revealed that the oyster G(s)alpha mRNA is widely expressed in a variety of tissues, with highest levels in the outer fold of mantle and epithelia of gill, the regions essential for biomineralization. We also show that overexpression of the pfG(s)alpha in mammalian MC3T3-E1 cells resulted in increased cAMP levels. Mutant pfG(s)alpha that has impaired CTX substrate diminished its ability to induce cAMP production. Furthermore, the alkaline phosphatase (ALP) activity, an indicator for mineralization, is induced by the G(s)alpha in MC3T3-E1 cells. These results indicated that G(s)alpha may be involved in regulation of physiological function, particularly in biological biomineralization.

Purification and Characterization of a Vaterite-Inducing Peptide, Pelovaterin, from the Eggshells ofPelodiscussinensis(Chinese Soft-Shelled Turtle)

Proteins play a crucial role in the biomineralization of hard tissues such as eggshells. We report here the purification, characterization, and in vitro mineralization studies of a peptide, pelovaterin, extracted from eggshells of a soft-shelled turtle. It is a glycine-rich peptide with 42 amino acid residues and three disulfide bonds. When tested in vitro, the peptide induced the formation of a metastable vaterite phase. The floret-shaped morphology formed at a lower concentration ( approximately 1 microM) was transformed into spherical particles at higher concentrations (>500 microM). The solution properties of the peptide are investigated by circular dichroism (CD), fluorescence emission spectroscopy, and dynamic light scattering (DLS) experiments. The conformation of pelovaterin changed from an unordered state at a low concentration to a beta-sheet structure at high concentrations. Fluorescence emission studies indicated that the quantum yield is significantly decreased at higher concentrations, accompanied by a blue shift in the emission maximum. At higher concentrations a red-edge excitation shift was observed, indicating the restricted mobility of the peptide. On the basis of these observations, we discuss the presence of a peptide concentration-dependent monomer-multimer equilibrium in solution and its role in controlling the nucleation, growth, and morphology of CaCO(3) crystals. This is the first peptide known to induce the nucleation and stabilization of the vaterite phase in solution.

The Carbonic Anhydrase Domain Protein Nacrein is Expressed in the Epithelial Cells of the Mantle and Acts as a Negative Regulator in Calcification in the Mollusc Pinctada fucata

Signals and organic matrix proteins secreted from the mantle are critical for the development of shells in molluscs. Nacrein, which is composed of a carbonic anhydrase domain and a Gly-X-Asn repeat domain, is one of the organic matrix proteins that accumulates in shells. In situ hybridization revealed that nacrein was expressed in the outer epithelial cells of the mantle of the pearl oyster Pinctada fucata. The recombinant nacrein protein inhibited the precipitation of calcium carbonate from a saturated solution containing CaCl2 and NaHCO3, indicating that it can act as a negative regulator for calcification in the shells of molluscs. Because deletion of the Gly-X-Asn repeat domain of nacrein had a significant effect on the ability of nacrein to inhibit the precipitation of calcium carbonate, it is conceivable that the repeat domain has a primary role in the inhibitory function of nacrein in shell formation. Together these studies suggest that nacrein functions as a negative regulator in calcification in the extrapallial space between the shell and the mantle by inhibiting the precipitation of CaCO3.

Asprich: A Novel Aspartic Acid-Rich Protein Family from the Prismatic Shell Matrix of the Bivalve Atrina rigida

Almost all mineralized tissues contain proteins that are unusually acidic. As they are also often intimately associated with the mineral phase, they are thought to fulfill important functions in controlling mineral formation. Relatively little is known about these important proteins, because their acidic nature causes technical difficulties during purification and characterization procedures. Much effort has been made to overcome these problems, particularly in the study of mollusk-shell formation. To date about 16 proteins from mollusk-shell organic matrices have been sequenced, but only two are unusually rich in aspartic and glutamic acids. Here we screened a cDNA library made from the mRNA of the shell-forming cells of a bivalve, Atrina rigida, using probes for short Asp-containing repeat sequences, and identified ten different proteins. Using more specific probes designed from one subgroup of conserved sequences, we obtained the full sequences of a family of seven aspartic acid-rich proteins, which we named "Asprich"; a subfamily of the unusually acidic shell-matrix proteins. Polyclonal antibodies raised against a synthetic peptide of the conserved acidic1 domain of these proteins reacted specifically with the matrix components of the calcitic prismatic layer, but not with those of the aragonitic nacreous layer. Thus the Asprich proteins are constituents of the prismatic layer shell matrix. We can identify different domains within these sequences, including a signal peptide characteristic of proteins destined for extracellular secretion, a conserved domain rich in aspartic acid that contains a sequence very similar to the calcium-binding domain of Calsequestrin, and another domain rich in aspartic acid, that varies between the seven sequences. We also identified a domain with DEAD repeats that may have Mg-binding capabilities. Although we do not know, as yet, the function of these proteins, their generally conserved sequences do indicate that they might well fulfill basic functions in shell formation.

Zona localization of shell matrix proteins in mantle of Haliotis tuberculata (Mollusca, Gastropoda)

Organic matrix from molluscan shells has the potential to regulate calcium carbonate deposition and crystallization. Control of crystal growth thus seems to depend on control of matrix protein secretion or activation processes in the mantle cells, about which little is known. Biomineralization is a highly orchestrated biological process. The aim of this work was to provide information about the source of shell matrix macromolecule production, within the external epithelium of the mantle. An in vivo approach was chosen to describe the histologic changes in the outer epithelium and in blood sinus distribution, associated with mantle cells implicated in shell matrix production. Our results characterized a topographic and time-dependent zonation of matrix proteins involved in shell biomineralization in the mantle of Haliotis.

Crystal Structure of Ovocleidin-17, a Major Protein of the Calcified Gallus gallus Eggshell

Ovocleidin-17 (OC17) from Gallus gallus is one of the best candidates to control and regulate the deposition of calcium carbonate in the calcified eggshell layer. Here, the crystal structure of monomeric OC17, determined at a resolution of 1.5 A, was refined to a crystallographic R-factor of 20.1%. This is the first protein directly involved in a non-pathological biomineralization process resolved by x-ray diffraction to date. The protein has a mixed alpha/beta structure containing a single C-type lectin-like domain. However, although OC17 shares the conserved scaffold of the C-type lectins, it does not bind carbohydrates. Nevertheless, in vitro OC17 modifies the crystalline habit of calcium carbonate (CaCO3) and the pattern of crystal growth at intervals of 5-200 microg/ml. Determining the three-dimensional structure of OC17 contributes to a better understanding of the biological behavior of structurally related biomolecules and of the mechanisms involved in eggshell and other mineralization processes.

C-type lectin-like domains in Fugu rubripes

Background

Members of the C-type lectin domain (CTLD) superfamily are metazoan proteins functionally important in glycoprotein metabolism, mechanisms of multicellular integration and immunity. Three genome-level studies on human, C. elegans and D. melanogaster reported previously demonstrated almost complete divergence among invertebrate and mammalian families of CTLD-containing proteins (CTLDcps).

Results

We have performed an analysis of CTLD family composition in Fugu rubripes using the draft genome sequence. The results show that all but two groups of CTLDcps identified in mammals are also found in fish, and that most of the groups have the same members as in mammals. We failed to detect representatives for CTLD groups V (NK cell receptors) and VII (lithostathine), while the DC-SIGN subgroup of group II is overrepresented in Fugu. Several new CTLD-containing genes, highly conserved between Fugu and human, were discovered using the Fugu genome sequence as a reference, including a CSPG family member and an SCP-domain-containing soluble protein. A distinct group of soluble dual-CTLD proteins has been identified, which may be the first reported CTLDcp group shared by invertebrates and vertebrates. We show that CTLDcp-encoding genes are selectively duplicated in Fugu, in a manner that suggests an ancient large-scale duplication event. We have verified 32 gene structures and predicted 63 new ones, and make our annotations available through a distributed annotation system (DAS) server and their sequences as additional files with this paper.

Conclusions

The vertebrate CTLDcp family was essentially formed early in vertebrate evolution and is completely different from the invertebrate families. Comparison of fish and mammalian genomes revealed three groups of CTLDcps and several new members of the known groups, which are highly conserved between fish and mammals, but were not identified in the study using only mammalian genomes. Despite limitations of the draft sequence, the Fugu rubripes genome is a powerful instrument for gene discovery and vertebrate evolutionary analysis. The composition of the CTLDcp superfamily in fish and mammals suggests that large-scale duplication events played an important role in the evolution of vertebrates.

Structure and expression of an unusually acidic matrix protein of pearl oyster shells

We report identification and characterization of the unusually acidic molluscan shell matrix protein Aspein, which may have important roles in calcium carbonate biomineralization. The Aspein gene (aspein) encodes a sequence of 413 amino acids, including a high proportion of Asp (60.4%), Gly (16.0%), and Ser (13.2%), and the predicted isoelectric point is 1.45; this is the most acidic of all the molluscan shell matrix proteins sequenced so far, or probably even of all known proteins on earth. The main body of Aspein is occupied by (Asp)(2-10) sequences punctuated with Ser-Gly dipeptides. RT-PCR demonstrated that the transcript of aspein is expressed at the outer edge of the mantle, corresponding to the calcitic prismatic layer, but not at the inner part of the mantle, corresponding to the aragonitic nacreous layer. Our findings and previous in vitro experiments taken together suggest that Aspein is responsible for directed formation of calcite in the shell of the pearl oyster Pinctada fucata.

Characterization of two molluscan crystal-modulating biomineralization proteins and identification of putative mineral binding domains

Ethylenediamine-tetraacetic acid extracted water-soluble matrix proteins in molluscan shells secreted from the mantle epithelia are believed to control crystal nucleation, morphology, orientation, and phase of the deposited mineral. Previously, atomic force microscopy demonstrated that abalone nacre proteins bind to growing step edges and to specific crystallographic faces of calcite, suggesting that inhibition of calcite growth may be one of the molecular processes required for growth of the less thermodynamically stable aragonite phase. Previous experiments were done with protein mixtures. To elucidate the role of single proteins, we have characterized two proteins isolated from the aragonitic component of nacre of the red abalone, Haliotis rufescens. These proteins, purified by hydrophobic interaction chromatography, are designated AP7 and AP24 (aragonitic protein of molecular weight 7 kDa and 24 kDa, respectively). Degenerate oligonucleotide primers corresponding to N-terminal and internal peptide sequences were used to amplify cDNA clones by a polymerase chain reaction from a mantle cDNA library; the deduced primary amino acid sequences are presented. Preliminary crystal growth experiments demonstrate that protein fractions enriched in AP7 and AP24 produced CaCO(3) crystals with morphology distinct from crystals grown in the presence of the total mixture of soluble aragonite-specific proteins. Peptides corresponding to the first 30 residues of the N-terminal sequences of both AP7 and AP24 were generated. The synthetic peptides frustrate the progression of step edges of a growing calcite surface, indicating that sequence features within the N-termini of AP7 and AP24 include domains that interact with CaCO(3). CD analyses demonstrate that the N-terminal peptide sequences do not possess significant percentages of alpha-helix or beta-strand secondary structure in solution. Instead, in both the presence and absence of Ca(II), the peptides retain unfolded conformations that may facilitate protein-mineral interaction.