presence of a glycerol-based buffer. Following this rationale, it could be
demonstrated that silicatein exists not only in the axial canal but also in the
extraspicular and extracellular space (M€ uller et al. 2005; Schr€ oder et al. 2006).
The enzymatic reaction mechanism of silicatein had been proposed by Cha et al.
(1999); the detailed properties of the reaction kinetics have been specified experimentally (M€ uller et al. 2008b).
9.8 Catabolic Enzyme: Silicase
In the course to further elucidate the metabolism of siliceous spicules, another
enzyme, silicase, was identified in the marine sponge S. domuncula. Silicase is able
to depolymerize amorphous silica. The cDNA was isolated and the deduced
ä
Fig. 9.9 (continued) constructed. The hitherto known three hexactinellid sequences were
included; silicatein from Crateromorpha meyeri (SILCA_CRATEROMORPHA; AM920776)
and from Monorhaphis chuni (SILCAa_MONORHAPHIS; FN394978) and the silicatein-like
protein Aulosaccus sp. (SILCA_AULOSACCUS; ACU86976.1). The bulk of silicatein sequences
has been identified in demosponges. First, the silicatein-a sequences from Suberites domuncula
(SILCAa_SUBERITES; CAC03737.1), Tethya aurantium (SILCAa_TETHYA; AAC23951.1),
Geodia cydonium (SILCAa_GEODIA; CAM57981.1) and Acanthodendrilla sp. Vietnam
(SILCAa_ACANTHODENDRILLA; ACH92669.1), as well as from Lubomirskia baicalensis
(SILCAa2_LUBOMIRSKIA; AJ968945) and from Ephydatia fluviatilis (SILCA_EPHYDATIA;
BAE54434.1). Second, the silicatein-b sequences from Suberites domuncula (SILCAb_SUBERITES;
CAH04635.1), Tethya aurantium (SILCAb_TETHYA; AF098670_1) and Acanthodendrilla sp.
Vietnam (SILCAb_ACANTHODENDRILLA; FJ013043.1). Third, silicateins that had been identified
in marine sponges from which only one isoform had been obtained; silicatein from Petrosia ficiformis
(SILCA_PETROSIA; AAO23671.1) and from Halichondria okadai (SILCA_HALICHONDRIA;
BAB86343.1). As reflected in the rooted tree, these silicateins derived from the cathepsins, among
which in this tree, the following sequences have been included; cathepsin-like protein 2 Crateromorpha
meyeri (CATL2_CRATEROMORPHA; CAP17585.1), cathepsin-like protein 1 (Crateromorpha
meyeri) (CATL1_CRATEROMORPHA; CAP17584.1), mRNA for cathepsin L (catl gene)
Aphrocallistes vastus (CATL_APHROCALLISTES); AJ968951cathepsin B Suberites domuncula
(CATLB_SUBERITES; CAH04630.1), cathepsin X/O Suberites domuncula (CATLX/
O_SUBERITES; |CAH04633.1). The resulting tree was rooted with the sequence from the papainlike cysteine peptidase XBCP3 Arabidopsis thaliana (PAPAIN_ARABIDOPSIS; AF388175_1).
In addition, the cathepsins from choanoflagellates had been included to show that they derived,
according to this tree, from the sponge cathepsins; the cysteine protease from Proterospongia sp.
(CAT_MONOSIGA ovata), the cathepsin from Monosiga brevicollis (CATP_MONOSIGA brevi) and
the cathepsin from Monosiga ovata MNL00000103. (b) A selected set of silicateins, the silicatein-a
from S. domuncula (SILICAa_SUBDO), and T. aurantium (SILICAa_TETHY), as well as the
silicatein-b from S. domuncula (SILICAb_SUBDO), and T. aurantium (SILICAb_TETHY) had
been included together with cathepsin L from S. domuncula (CATLL_SUBDO). Residues conserved
(similar or related with respect to their physicochemical properties) in all sequences are shown in white
on black, and those in at least five sequences in black on gray. The characteristic sites in the sequences
are marked; the catalytic triad amino acids, Ser in silicateins and Cys in cathepsin, and His and Asn.
The borders of the signal peptide (signal), the propeptide (propeptide) and the mature silicatein
(mature) are given
9 The Unique Invention of the Siliceous Sponges
271
demonstrated that silicatein exists not only in the axial canal but also in the
extraspicular and extracellular space (M€ uller et al. 2005; Schr€ oder et al. 2006).
The enzymatic reaction mechanism of silicatein had been proposed by Cha et al.
(1999); the detailed properties of the reaction kinetics have been specified experimentally (M€ uller et al. 2008b).
9.8 Catabolic Enzyme: Silicase
In the course to further elucidate the metabolism of siliceous spicules, another
enzyme, silicase, was identified in the marine sponge S. domuncula. Silicase is able
to depolymerize amorphous silica. The cDNA was isolated and the deduced
ä
Fig. 9.9 (continued) constructed. The hitherto known three hexactinellid sequences were
included; silicatein from Crateromorpha meyeri (SILCA_CRATEROMORPHA; AM920776)
and from Monorhaphis chuni (SILCAa_MONORHAPHIS; FN394978) and the silicatein-like
protein Aulosaccus sp. (SILCA_AULOSACCUS; ACU86976.1). The bulk of silicatein sequences
has been identified in demosponges. First, the silicatein-a sequences from Suberites domuncula
(SILCAa_SUBERITES; CAC03737.1), Tethya aurantium (SILCAa_TETHYA; AAC23951.1),
Geodia cydonium (SILCAa_GEODIA; CAM57981.1) and Acanthodendrilla sp. Vietnam
(SILCAa_ACANTHODENDRILLA; ACH92669.1), as well as from Lubomirskia baicalensis
(SILCAa2_LUBOMIRSKIA; AJ968945) and from Ephydatia fluviatilis (SILCA_EPHYDATIA;
BAE54434.1). Second, the silicatein-b sequences from Suberites domuncula (SILCAb_SUBERITES;
CAH04635.1), Tethya aurantium (SILCAb_TETHYA; AF098670_1) and Acanthodendrilla sp.
Vietnam (SILCAb_ACANTHODENDRILLA; FJ013043.1). Third, silicateins that had been identified
in marine sponges from which only one isoform had been obtained; silicatein from Petrosia ficiformis
(SILCA_PETROSIA; AAO23671.1) and from Halichondria okadai (SILCA_HALICHONDRIA;
BAB86343.1). As reflected in the rooted tree, these silicateins derived from the cathepsins, among
which in this tree, the following sequences have been included; cathepsin-like protein 2 Crateromorpha
meyeri (CATL2_CRATEROMORPHA; CAP17585.1), cathepsin-like protein 1 (Crateromorpha
meyeri) (CATL1_CRATEROMORPHA; CAP17584.1), mRNA for cathepsin L (catl gene)
Aphrocallistes vastus (CATL_APHROCALLISTES); AJ968951cathepsin B Suberites domuncula
(CATLB_SUBERITES; CAH04630.1), cathepsin X/O Suberites domuncula (CATLX/
O_SUBERITES; |CAH04633.1). The resulting tree was rooted with the sequence from the papainlike cysteine peptidase XBCP3 Arabidopsis thaliana (PAPAIN_ARABIDOPSIS; AF388175_1).
In addition, the cathepsins from choanoflagellates had been included to show that they derived,
according to this tree, from the sponge cathepsins; the cysteine protease from Proterospongia sp.
(CAT_MONOSIGA ovata), the cathepsin from Monosiga brevicollis (CATP_MONOSIGA brevi) and
the cathepsin from Monosiga ovata MNL00000103. (b) A selected set of silicateins, the silicatein-a
from S. domuncula (SILICAa_SUBDO), and T. aurantium (SILICAa_TETHY), as well as the
silicatein-b from S. domuncula (SILICAb_SUBDO), and T. aurantium (SILICAb_TETHY) had
been included together with cathepsin L from S. domuncula (CATLL_SUBDO). Residues conserved
(similar or related with respect to their physicochemical properties) in all sequences are shown in white
on black, and those in at least five sequences in black on gray. The characteristic sites in the sequences
are marked; the catalytic triad amino acids, Ser in silicateins and Cys in cathepsin, and His and Asn.
The borders of the signal peptide (signal), the propeptide (propeptide) and the mature silicatein
(mature) are given
9 The Unique Invention of the Siliceous Sponges
271