adjoining the mantle of mollusks utilizes similar processes. These generalizations,
while providing a useful viewpoint, do not easily lead to experimentally testable
models. Just what are the matrix molecules? How do they participate? What
molecules preside over regulation of the transition from ACC to calcite or aragonite? Of amorphous calcium phosphates to carbonated apatite? How is the secretion
of molecules linked to attaining particular crystalline polymorphs?
New facts have emerged that may help inform some models. In our judgment,
these are:
1. Amorphous forms of the mineral can and do serve as precursors,
2. Very large numbers of protein-modifying enzymes are occluded in the amorphous mineral and could regulate transitions to crystalline state, polymorph
selection, resistance to fracture, and other properties of the crystal,
3. While there are few, if any, orthologs of occluded “matrix” proteins present in
shells, stereoms, or vertebrate teeth and bones, proteins with intrinsically disordered domains play a crucial role in regulation of nascent mineralization.
We have already touched on the first point, and while the presence of an
amorphous precursor has not been shown to be near-universal, the examples have
been reported from three different phyla: echinoderms (Beniash et al. 1997; Politi
et al. 2004, 2008; Killian et al. 2009), mollusks (Weiss et al. 2002; Nassif et al.
2005), and chordates (Mahamid et al. 2008; Beniash et al. 2009). Thus the idea of
generalizing amorphous precursors is not unreasonable. Indeed, it helps explain
how crystalline biominerals can attain the wide variety of shapes that they do. It
should be noted, however, that Kudo et al. (2010) did not observe ACC in nacre
deposition in the Japanese oyster, Crassostrea nippona.
The second point is hinted at in the recent proteomic studies of echinoderm teeth,
test, and spines. An extraordinary number of different proteins are occluded; many
of them are present in amounts that are unlikely to be due to contamination, and
indeed, were identified earlier by methods with less resolution (Killian and Wilt,
1996). Of course, the presence of some organic components could be due to
contamination. Mann et al. (2008b) did compare protein content of ground tooth
fragments with and without treatment with NaOCl. The intentional “contamination”
indicated that many of the proteins provisionally classified as “occluded” (such
as carbonic anhydrase, metalloproteases, cyclophilins) are probably authentic
occluded proteins, although some very rare ones (e.g., histones) are more likely
contaminants. Therefore, the putative presence of proteases, e.g., indicates that postsecretory modifications of matrix proteins can and probably do occur, and thus,
could be expected to participate in the regulation of changes in the mineral phase.
Third, and finally, there is now enough genomic information from vertebrates,
from the sea urchin, and from some mollusks and invertebrates, to prudently
conclude that major matrix proteins from one clade are not present in others. For
instance, the SM30 and SM50 protein families of echinoderms are not found in
mollusks or vertebrates. Dentinal phosphoproteins of vertebrate teeth are not found
in echinoderms or mollusks, and so on. What does emerge, however, is the presence
of proteins in mollusks, (AP24, Pif, n16, etc.), echinoderms, and vertebrates
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P.U.P.A Gilbert and F.H. Wilt
while providing a useful viewpoint, do not easily lead to experimentally testable
models. Just what are the matrix molecules? How do they participate? What
molecules preside over regulation of the transition from ACC to calcite or aragonite? Of amorphous calcium phosphates to carbonated apatite? How is the secretion
of molecules linked to attaining particular crystalline polymorphs?
New facts have emerged that may help inform some models. In our judgment,
these are:
1. Amorphous forms of the mineral can and do serve as precursors,
2. Very large numbers of protein-modifying enzymes are occluded in the amorphous mineral and could regulate transitions to crystalline state, polymorph
selection, resistance to fracture, and other properties of the crystal,
3. While there are few, if any, orthologs of occluded “matrix” proteins present in
shells, stereoms, or vertebrate teeth and bones, proteins with intrinsically disordered domains play a crucial role in regulation of nascent mineralization.
We have already touched on the first point, and while the presence of an
amorphous precursor has not been shown to be near-universal, the examples have
been reported from three different phyla: echinoderms (Beniash et al. 1997; Politi
et al. 2004, 2008; Killian et al. 2009), mollusks (Weiss et al. 2002; Nassif et al.
2005), and chordates (Mahamid et al. 2008; Beniash et al. 2009). Thus the idea of
generalizing amorphous precursors is not unreasonable. Indeed, it helps explain
how crystalline biominerals can attain the wide variety of shapes that they do. It
should be noted, however, that Kudo et al. (2010) did not observe ACC in nacre
deposition in the Japanese oyster, Crassostrea nippona.
The second point is hinted at in the recent proteomic studies of echinoderm teeth,
test, and spines. An extraordinary number of different proteins are occluded; many
of them are present in amounts that are unlikely to be due to contamination, and
indeed, were identified earlier by methods with less resolution (Killian and Wilt,
1996). Of course, the presence of some organic components could be due to
contamination. Mann et al. (2008b) did compare protein content of ground tooth
fragments with and without treatment with NaOCl. The intentional “contamination”
indicated that many of the proteins provisionally classified as “occluded” (such
as carbonic anhydrase, metalloproteases, cyclophilins) are probably authentic
occluded proteins, although some very rare ones (e.g., histones) are more likely
contaminants. Therefore, the putative presence of proteases, e.g., indicates that postsecretory modifications of matrix proteins can and probably do occur, and thus,
could be expected to participate in the regulation of changes in the mineral phase.
Third, and finally, there is now enough genomic information from vertebrates,
from the sea urchin, and from some mollusks and invertebrates, to prudently
conclude that major matrix proteins from one clade are not present in others. For
instance, the SM30 and SM50 protein families of echinoderms are not found in
mollusks or vertebrates. Dentinal phosphoproteins of vertebrate teeth are not found
in echinoderms or mollusks, and so on. What does emerge, however, is the presence
of proteins in mollusks, (AP24, Pif, n16, etc.), echinoderms, and vertebrates
218
P.U.P.A Gilbert and F.H. Wilt