7.5.2 Matrix Proteins of the Tooth
The tooth elements are also known to contain organic material occluded in the
calcite. The entire mineralized portion of the tooth, ground and cleaned of adherent
material with NaOCl, has been subjected to proteomic analysis by Mann et al.
(2008b). Some 138 proteins were identified, 56 of which had been previously
identified by this group in test and spine. Major components were identical to
abundant proteins in test and spine, while some apparent tooth-specific proteins
were especially rich in alanine and proline.
Killian et al. (2010) using PCR methods demonstrated that mRNA encoding
SM30 E was very prominent both in mineralized portions and the plumula,
and earlier work showed that SM50 is present. Recent work by Veis and
his collaborators (Alvares et al., 2009) has identified a number of occluded
phosphoproteins. Two of them are identical to proteins identified in an EST library
of PMCs of the embryo, notably P16 and P19. P16 had been shown by Cheers and
Ettensohn (2005) to be a transmembrane protein whose function was essential for
embryonic spicule deposition. Alvares et al. (2009) showed using specific antibody
staining that P16, dubbed UTMP16 in the tooth, is found in syncytial membranes in
contact with mineral. Mann et al. (2010) have recently analyzed the tooth proteome
for the presence of phosphoproteins; they found 15 phosphorylated proteins, 13 of
which are unique to tooth tissue.
One of these, named phosphodontin, is rather prominent and contains 35 repeats
of an acidic 11–12 amino acid motif that is phosphorylated. Though the amino acid
sequence is not orthologous to any known vertebrate tooth protein, the sequence
and charge of the peptides indicate it is probably an intrinsically disordered protein.
7.6 Generalizations
Biomineralization processes in Echinoderms display certain general features. Mineralization proceeds in privileged spaces closely apposed by cellular processes, i.e.,
surrounded by phospholipid membranes. Large numbers of proteins, many of which
(but not all) are acidic glycoproteins, are secreted into this space, and ACC, perhaps
in the hydrated form, is also deposited into the same places. The ACC gradually
transforms to calcite, and some of the proteins are occluded within the forming
skeletal element. The details of initial assembly of ACC and protein are not clear,
but the slow conversion of ACC to calcite probably occurs by some atomic level
reorientation of anhydrous ACC to calcite via a secondary nucleation type of
propagation, presenting, at least temporarily, a patchwork of ACC and calcite.
In some respects, this is not so different from proposals made by others (Veis
2008): in this view, cells construct a structural matrix in a defined space, other
molecules then assist in orderly nucleation and regulation of crystal growth, habit,
shape, and size. One could argue that formation of shells in the extrapallial space
7 Molecular Aspects of Biomineralization of the Echinoderm Endoskeleton
217
The tooth elements are also known to contain organic material occluded in the
calcite. The entire mineralized portion of the tooth, ground and cleaned of adherent
material with NaOCl, has been subjected to proteomic analysis by Mann et al.
(2008b). Some 138 proteins were identified, 56 of which had been previously
identified by this group in test and spine. Major components were identical to
abundant proteins in test and spine, while some apparent tooth-specific proteins
were especially rich in alanine and proline.
Killian et al. (2010) using PCR methods demonstrated that mRNA encoding
SM30 E was very prominent both in mineralized portions and the plumula,
and earlier work showed that SM50 is present. Recent work by Veis and
his collaborators (Alvares et al., 2009) has identified a number of occluded
phosphoproteins. Two of them are identical to proteins identified in an EST library
of PMCs of the embryo, notably P16 and P19. P16 had been shown by Cheers and
Ettensohn (2005) to be a transmembrane protein whose function was essential for
embryonic spicule deposition. Alvares et al. (2009) showed using specific antibody
staining that P16, dubbed UTMP16 in the tooth, is found in syncytial membranes in
contact with mineral. Mann et al. (2010) have recently analyzed the tooth proteome
for the presence of phosphoproteins; they found 15 phosphorylated proteins, 13 of
which are unique to tooth tissue.
One of these, named phosphodontin, is rather prominent and contains 35 repeats
of an acidic 11–12 amino acid motif that is phosphorylated. Though the amino acid
sequence is not orthologous to any known vertebrate tooth protein, the sequence
and charge of the peptides indicate it is probably an intrinsically disordered protein.
7.6 Generalizations
Biomineralization processes in Echinoderms display certain general features. Mineralization proceeds in privileged spaces closely apposed by cellular processes, i.e.,
surrounded by phospholipid membranes. Large numbers of proteins, many of which
(but not all) are acidic glycoproteins, are secreted into this space, and ACC, perhaps
in the hydrated form, is also deposited into the same places. The ACC gradually
transforms to calcite, and some of the proteins are occluded within the forming
skeletal element. The details of initial assembly of ACC and protein are not clear,
but the slow conversion of ACC to calcite probably occurs by some atomic level
reorientation of anhydrous ACC to calcite via a secondary nucleation type of
propagation, presenting, at least temporarily, a patchwork of ACC and calcite.
In some respects, this is not so different from proposals made by others (Veis
2008): in this view, cells construct a structural matrix in a defined space, other
molecules then assist in orderly nucleation and regulation of crystal growth, habit,
shape, and size. One could argue that formation of shells in the extrapallial space
7 Molecular Aspects of Biomineralization of the Echinoderm Endoskeleton
217