typical silk-like framework protein. However, poly(glycine) is suggested not only
to participate in the formation of crystalline b-sheets, but also to bind calcium ions
(Sudo et al. 1997). Gly-X-Asn repeat domain of Nacrein is linked to the inhibition
of calcium carbonate precipitation (Miyamoto et al. 1996).
Acting as a Protein Complex
The presence of a protein complex is a prerequisite for aragonite crystallization
(Matsushiro et al. 2003). Only the complex of pearlin and pearl keratin, and the
reconstituted complex, induces aragonite crystallization in vitro within a calciumcarbonate-saturated solution containing Mg
2+ , whereas the mixture of the individual components has no function. Other known protein complexes present in the
nacreous layer of the pearl oyster Pinctada, include P60 (complex of Nacrein and
its two derivatives, N28 and N35, by disulfide bridges) (Lao et al. 2007) and Pif.
The latter, a complex of Pif 97and Pif 80, is assumed to assemble with N16 and
other proteins to control nacre formation (Suzuki et al. 2009). Because the proteins
always form complexes, it is difficult to purify them from nacreous layers. Characterization of the protein complexes will facilitate an understanding of their
functions.
Posttranslational Modifications
Matrix proteins usually exhibit several posttranslational modifications, such as
phosphorylations, glycosylations, and sulfations, etc. (Marin et al. 2008). Both
Nacrein and Pearlin are sulfated glycoproteins, the former containing N-linked
sialic acid at its terminus (Takakura et al. 2008), whereas the latter has
mucopolysaccharides (Miyashita et al. 2000). ACCBP (amorphous calcium carbonate binding protein), purified from extrapallial fluid of Pindata fucata, has two
potential glycosylation sites at Asn29 and Asn184 (Ma et al. 2007), and is heavily
glycosylated with peculiar glycans (data not shown). These posttranslational
modifications may have important roles in nacre biomineralization. Structured
polysaccharide moieties of glycoproteins are putatively important in controlling
the crystal growth in vivo (Albeck et al. 1996), while the negative charge on sulfate
groups linked to the mucopolysaccharide of Pearlin is assumed to bind to calcium
(Miyashita et al. 2000). The covalently bound sulfated polysaccharides supposedly
act as “antennae” that help to accumulate sufficient numbers of Ca
2+ ions around
the asp-rich binding/nucleation sites on the antiparallel b–pleated sheet of the
matrix (Mann 2001). Nevertheless, glycosylations of ACCBP are also a prerequisite for performing functions in the control of nacre morphology (data not shown).
Matrix protein Pif regulates nacre formation by its posttranslational modification
products, Pif 97 and Pif 80 (Suzuki et al. 2009). These kinds of posttranslational
modifications probably exist in other nacreous matrix proteins, and it is necessary to
investigate their functions in nacre biomineralization.
338
L.-p. Xie et al.
to participate in the formation of crystalline b-sheets, but also to bind calcium ions
(Sudo et al. 1997). Gly-X-Asn repeat domain of Nacrein is linked to the inhibition
of calcium carbonate precipitation (Miyamoto et al. 1996).
Acting as a Protein Complex
The presence of a protein complex is a prerequisite for aragonite crystallization
(Matsushiro et al. 2003). Only the complex of pearlin and pearl keratin, and the
reconstituted complex, induces aragonite crystallization in vitro within a calciumcarbonate-saturated solution containing Mg
2+ , whereas the mixture of the individual components has no function. Other known protein complexes present in the
nacreous layer of the pearl oyster Pinctada, include P60 (complex of Nacrein and
its two derivatives, N28 and N35, by disulfide bridges) (Lao et al. 2007) and Pif.
The latter, a complex of Pif 97and Pif 80, is assumed to assemble with N16 and
other proteins to control nacre formation (Suzuki et al. 2009). Because the proteins
always form complexes, it is difficult to purify them from nacreous layers. Characterization of the protein complexes will facilitate an understanding of their
functions.
Posttranslational Modifications
Matrix proteins usually exhibit several posttranslational modifications, such as
phosphorylations, glycosylations, and sulfations, etc. (Marin et al. 2008). Both
Nacrein and Pearlin are sulfated glycoproteins, the former containing N-linked
sialic acid at its terminus (Takakura et al. 2008), whereas the latter has
mucopolysaccharides (Miyashita et al. 2000). ACCBP (amorphous calcium carbonate binding protein), purified from extrapallial fluid of Pindata fucata, has two
potential glycosylation sites at Asn29 and Asn184 (Ma et al. 2007), and is heavily
glycosylated with peculiar glycans (data not shown). These posttranslational
modifications may have important roles in nacre biomineralization. Structured
polysaccharide moieties of glycoproteins are putatively important in controlling
the crystal growth in vivo (Albeck et al. 1996), while the negative charge on sulfate
groups linked to the mucopolysaccharide of Pearlin is assumed to bind to calcium
(Miyashita et al. 2000). The covalently bound sulfated polysaccharides supposedly
act as “antennae” that help to accumulate sufficient numbers of Ca
2+ ions around
the asp-rich binding/nucleation sites on the antiparallel b–pleated sheet of the
matrix (Mann 2001). Nevertheless, glycosylations of ACCBP are also a prerequisite for performing functions in the control of nacre morphology (data not shown).
Matrix protein Pif regulates nacre formation by its posttranslational modification
products, Pif 97 and Pif 80 (Suzuki et al. 2009). These kinds of posttranslational
modifications probably exist in other nacreous matrix proteins, and it is necessary to
investigate their functions in nacre biomineralization.
338
L.-p. Xie et al.