Distribution Differences
Different genes that encode nacreous and prismatic matrix proteins are expressed in
different regions of the mantle (Takeuchi and Endo 2006), and even in different
developmental stage of the larva shell (Miyazaki et al. 2010). Matrix proteins are
distributed in different layers of the shell or in different regions in the same layer,
which is related to their functions. For instance, proteins involved in aragonitic
formation occur in the nacreous layer of the shell, whereas proteins involved in
calcite formation are found in prismatic layers (Falini et al. 1996). In each individual interlamellar sheet of the nacreous layer, proteins occur in different zones
underlying a single aragonite crystal, which means they have a different function.
Immunofluorescence staining has revealed that aragonite-nucleating fractions are
primarily localized in the center of the crystal imprint and the intertabular matrix of
interlamellar sheets (Addadi et al. 2006; Nudelman et al. 2006).
12.3.2.2 Framework Proteins
In the nacreous layer, the framework proteins are rich in Ala and Gly. They are
thought to fill the space between two interlamellar sheets in a hydrated gel-like state
before biomineralization (Nudelman et al. 2008). So far, only one silk-like framework matrix protein, MSI60, has been identified from the nacreous layers of the
pearl oyster Pinctada fucata. This protein participates in the formation of CaCO 3
crystals, presumably by binding soluble Asp-rich matrix glycoproteins and carbonic anhydrase (Sudo et al. 1997). Its messenger RNA is preferentially expressed
only in the more dorsal outer epithelia of the mantle (Takeuchi and Endo 2006) and
is highly expressed in the pearl sac (Wang et al. 2009) in which its relative
expression level is related to pearl quality (Inoue et al. 2010).
12.3.2.3 Regulative Proteins
Proteins from Pearl Oyster Nacre
Nacrein is the first identified molluskan matrix protein from which a complete
amino acid sequence has been obtained. It is thought to be specifically involved in
forming the nacreous layer of the pearl oysters Pinctada fucata (Miyamoto et al.
1996) and P. maxima (Kono et al. 2000). The protein, secreted by epithelial cells of
the mantle (Gong et al. 2008a, b), is a soluble matrix glycoprotein containing sulfite
and N-linked sialic acid (Takakura et al. 2008), whose transcript is expressed in
dorsal and ventral regions of the mantle (Takeuchi and Endo 2006), especially
in the epithelial cells (Miyamoto et al. 2005). Having a carbonic anhydrase domain,
Nacrein acts as an enzyme to catalyze HCO 3
À formation, thus participating in
calcium carbonate crystal formation of the nacreous layer (Miyamoto et al. 1996). It
has another acidic Gly-Xaa-Asn (Xaa ¼ Asp, Asn, or Glu) repeat domain, which
12 Molecular Approaches to Understand Biomineralization of Shell Nacreous Layer
339
Different genes that encode nacreous and prismatic matrix proteins are expressed in
different regions of the mantle (Takeuchi and Endo 2006), and even in different
developmental stage of the larva shell (Miyazaki et al. 2010). Matrix proteins are
distributed in different layers of the shell or in different regions in the same layer,
which is related to their functions. For instance, proteins involved in aragonitic
formation occur in the nacreous layer of the shell, whereas proteins involved in
calcite formation are found in prismatic layers (Falini et al. 1996). In each individual interlamellar sheet of the nacreous layer, proteins occur in different zones
underlying a single aragonite crystal, which means they have a different function.
Immunofluorescence staining has revealed that aragonite-nucleating fractions are
primarily localized in the center of the crystal imprint and the intertabular matrix of
interlamellar sheets (Addadi et al. 2006; Nudelman et al. 2006).
12.3.2.2 Framework Proteins
In the nacreous layer, the framework proteins are rich in Ala and Gly. They are
thought to fill the space between two interlamellar sheets in a hydrated gel-like state
before biomineralization (Nudelman et al. 2008). So far, only one silk-like framework matrix protein, MSI60, has been identified from the nacreous layers of the
pearl oyster Pinctada fucata. This protein participates in the formation of CaCO 3
crystals, presumably by binding soluble Asp-rich matrix glycoproteins and carbonic anhydrase (Sudo et al. 1997). Its messenger RNA is preferentially expressed
only in the more dorsal outer epithelia of the mantle (Takeuchi and Endo 2006) and
is highly expressed in the pearl sac (Wang et al. 2009) in which its relative
expression level is related to pearl quality (Inoue et al. 2010).
12.3.2.3 Regulative Proteins
Proteins from Pearl Oyster Nacre
Nacrein is the first identified molluskan matrix protein from which a complete
amino acid sequence has been obtained. It is thought to be specifically involved in
forming the nacreous layer of the pearl oysters Pinctada fucata (Miyamoto et al.
1996) and P. maxima (Kono et al. 2000). The protein, secreted by epithelial cells of
the mantle (Gong et al. 2008a, b), is a soluble matrix glycoprotein containing sulfite
and N-linked sialic acid (Takakura et al. 2008), whose transcript is expressed in
dorsal and ventral regions of the mantle (Takeuchi and Endo 2006), especially
in the epithelial cells (Miyamoto et al. 2005). Having a carbonic anhydrase domain,
Nacrein acts as an enzyme to catalyze HCO 3
À formation, thus participating in
calcium carbonate crystal formation of the nacreous layer (Miyamoto et al. 1996). It
has another acidic Gly-Xaa-Asn (Xaa ¼ Asp, Asn, or Glu) repeat domain, which
12 Molecular Approaches to Understand Biomineralization of Shell Nacreous Layer
339