lived, has to decay, and may live again in another form”. However, it took until
1999 when Cha et al. discovered that the main constituent of the proteinaceous
filament within the axial canal of spicules is an enzyme which might be involved in
biosilica formation, and that was consequently termed silicatein,. Soon after the
discovery of this anabolic enzyme, also the corresponding catabolic enzyme
(silicase) was detected (Schr€ oder et al. 2003). The identification of a biosilica
degrading enzyme supported the view that the siliceous components in spicules
are under metabolic control (Eckert et al. 2006). Studies on the metabolism of
spicules on the cellular level became possible after the introduction of a poriferan
cell culture system, primmorphs (Imsiecke et al. 1995; Custo ´dio et al. 1998).
Already, the first contribution on that topic resolved that spicule formation starts
intracellularly in “J” sclerocytes, by formation of an initial organic axial filament,
around which the inorganic silica mantel is deposited. This result had later been
corroborated by application of more advanced immunochemical and electron
microscopy techniques (M€ uller et al. 2007a).
After the discovery of the cathepsin L (cysteine protease)-related silicatein
(Shimizu et al. 1998; Cha et al. 1999) in spicules of the demosponge Tethya
aurantium, several related genes were elucidated in both marine and freshwater
demosponges (reviewed in M€ uller et al. 2007a). The corresponding deduced
polypeptides comprise about 325 amino acids (aa) with a molecular weight of ca.
35 kDa. During maturation, this primary translation product (proenzyme) is
processed by cleaving off a signal peptide (aa 1 to aa 17 ; S. domuncula [demosponge]
silicatein-a) and the adjacent propeptide (aa 18 to aa 112 ;), resulting in the mature
enzyme that has a size of 24–25 kDa. Similar to cathepsins, the catalytic center of
silicatein contains His and Asn. However, the Cys of the cathepsins’ catalytic triad
is exchanged by Ser in silicatein. In addition to about ten putative protein kinase
phosphorylation sites, silicateins display a cluster of serine residues that is found
close to the central aa residue of the catalytic triad, but is otherwise missing
in cathepsins. Subsequent phylogenetic analyses revealed that silicateins form a
separate branch from cathepsins (M€ uller et al. 2007a). The alignment and the
phylogenetic tree are given in Fig. 9.9a, b.
The difficult accessibility of hexactinellids, which live primarily in depths of
more than 300 m, generally results in a very poor sampling. Accordingly, only
recently the first hexactinellid silicatein (Crateromorpha meyeri) could be
identified and characterized (M€ uller et al. 2008c). This molecule shares high
similarity to the demosponge sequences (expect value of 8e
-58 ) and contains the
same catalytic triad amino acids. However, striking in the C. meyeri sequence is a
second Ser-rich cluster, which is located between the second and the third aa of
the catalytic triad; Fig. 9.9b. Strong binding of the protein to the spicule silica
surface has been attributed to this cluster (M€ uller et al. 2008a). The posttranslational modifications of silicatein have been found to be essential for the enzyme
activity with respect to (1) association with other structural and functional
molecules within the tissue and (2) self-association/self-assembly. For those studies, silicatein had been isolated from spicules in the absence of HF, but in the
9 The Unique Invention of the Siliceous Sponges
269
1999 when Cha et al. discovered that the main constituent of the proteinaceous
filament within the axial canal of spicules is an enzyme which might be involved in
biosilica formation, and that was consequently termed silicatein,. Soon after the
discovery of this anabolic enzyme, also the corresponding catabolic enzyme
(silicase) was detected (Schr€ oder et al. 2003). The identification of a biosilica
degrading enzyme supported the view that the siliceous components in spicules
are under metabolic control (Eckert et al. 2006). Studies on the metabolism of
spicules on the cellular level became possible after the introduction of a poriferan
cell culture system, primmorphs (Imsiecke et al. 1995; Custo ´dio et al. 1998).
Already, the first contribution on that topic resolved that spicule formation starts
intracellularly in “J” sclerocytes, by formation of an initial organic axial filament,
around which the inorganic silica mantel is deposited. This result had later been
corroborated by application of more advanced immunochemical and electron
microscopy techniques (M€ uller et al. 2007a).
After the discovery of the cathepsin L (cysteine protease)-related silicatein
(Shimizu et al. 1998; Cha et al. 1999) in spicules of the demosponge Tethya
aurantium, several related genes were elucidated in both marine and freshwater
demosponges (reviewed in M€ uller et al. 2007a). The corresponding deduced
polypeptides comprise about 325 amino acids (aa) with a molecular weight of ca.
35 kDa. During maturation, this primary translation product (proenzyme) is
processed by cleaving off a signal peptide (aa 1 to aa 17 ; S. domuncula [demosponge]
silicatein-a) and the adjacent propeptide (aa 18 to aa 112 ;), resulting in the mature
enzyme that has a size of 24–25 kDa. Similar to cathepsins, the catalytic center of
silicatein contains His and Asn. However, the Cys of the cathepsins’ catalytic triad
is exchanged by Ser in silicatein. In addition to about ten putative protein kinase
phosphorylation sites, silicateins display a cluster of serine residues that is found
close to the central aa residue of the catalytic triad, but is otherwise missing
in cathepsins. Subsequent phylogenetic analyses revealed that silicateins form a
separate branch from cathepsins (M€ uller et al. 2007a). The alignment and the
phylogenetic tree are given in Fig. 9.9a, b.
The difficult accessibility of hexactinellids, which live primarily in depths of
more than 300 m, generally results in a very poor sampling. Accordingly, only
recently the first hexactinellid silicatein (Crateromorpha meyeri) could be
identified and characterized (M€ uller et al. 2008c). This molecule shares high
similarity to the demosponge sequences (expect value of 8e
-58 ) and contains the
same catalytic triad amino acids. However, striking in the C. meyeri sequence is a
second Ser-rich cluster, which is located between the second and the third aa of
the catalytic triad; Fig. 9.9b. Strong binding of the protein to the spicule silica
surface has been attributed to this cluster (M€ uller et al. 2008a). The posttranslational modifications of silicatein have been found to be essential for the enzyme
activity with respect to (1) association with other structural and functional
molecules within the tissue and (2) self-association/self-assembly. For those studies, silicatein had been isolated from spicules in the absence of HF, but in the
9 The Unique Invention of the Siliceous Sponges
269