Identification of a silicatein(-related) protease in the giant spicules of the deep-sea hexactinellid Monorhaphis chuni.
2008
SUMMARY Silicateins, members of the cathepsin L family, are enzymes that have been
shown to be involved in the biosynthesis/condensation of biosilica in spicules
from Demospongiae (phylum Porifera), e.g. Tethya aurantium and
Suberites domuncula . The class Hexactinellida also forms spicules
from this inorganic material. This class of sponges includes species that form
the largest biogenic silica structures on earth. The giant basal spicules from
the hexactinellids Monorhaphis chuni and Monorhaphis
intermedia can reach lengths of up to 3 m and diameters of 10 mm. The
giant spicules as well as the tauactines consist of a biosilica shell that
surrounds the axial canal, which harbours the axial filament, in regular
concentric, lamellar layers, suggesting an appositional growth of the
spicules. The lamellae contain 27 kDa proteins, which undergo
post-translational modification (phosphorylation), while total spicule
extracts contain additional 70 kDa proteins. The 27 kDa proteins cross-reacted
with anti-silicatein antibodies. The extracts of spicules from the
hexactinellid Monorhaphis displayed proteolytic activity like the
silicateins from the demosponge S. domuncula . Since the proteolytic
activity in spicule extracts from both classes of sponge could be sensitively
inhibited by E-64 (a specific cysteine proteinase inhibitor), we used a
labelled E-64 sample as a probe to identify the protein that bound to this
inhibitor on a blot. The experiments revealed that the labelled E-64
selectively recognized the 27 kDa protein. Our data strongly suggest that
silicatein(-related) molecules are also present in Hexactinellida. These new
results are considered to also be of impact for applied biotechnological
studies.
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