Soluble proteins control growth of skeleton crystals in three coralline demosponges

Calcified demosponges (coralline sponges, sclerosponges), the first metazoa producing a carbonate skeleton, used to be important reef building organis ms in the past. The relatives of this group investigated here, Spirastrella (Acanthochaetetes) wellsi, Astrosclera willeyana and Vaceletia cf. crypta, are restricted to cryptic niches of modem Pacific coral reefs and may be c onsidered as ,,living fossils". They are characterized by a basic biologica lly controlled metazoan biomineralization process. Each of the investigated taxa forms its calcareous basal skeleton in a highly specialized way. More over, each taxon secretes distinct Ca2+-binding macromolecules which were e ntrapped within the calcium carbonate crystals during skeleton formation. T herefore these Ca2(+)-binding macromolecules were also described as intracr ystalline macromolecules. When isolated and separated by SDS polyacrylamide gel electrophoresis, the organic skeleton matrix of the three species reve aled to be composed of a respective distinct array of EDTA-soluble proteins . A single protein of 41 kDa was detected in S. wellsi, two proteins of 38 and 120 kDa in A. willeyana, and four proteins of 18 kDa, 30 kDa, 33 kDa, a nd 37 kDa in Vaceletia sp. When run on IEF gel, the Ca2+-binding proteins g ave staining bands at pH values between 5.25 and 5.65. As proved by an in v itro mineralization assay, the extracted proteins effectively inhibit CaCO3 and SrCO3 precipitation, respectively, in a saturated solution. Biochemica l properties and behavior of the extracted proteins strongly suggest that t hey are involved in crystal nucleation and skeleton carbonate formation wit hin the calcified sponges studied here.