The molecular mechanisms of the distinct calcium-dependent aggregation systems in marine sponges and corals.

During the last 15 years we have developed two biological systems, with whom it was possible to study the Ca++-dependent and the Ca++-independent adhesion on cellular level. In contrast to cells from other multicellular organisms, cells from the marine sponge Geodia cydonium are provided with Ca++-dependent adhesion mechanisms only. Two different mechanisms have been discovered by us, which were termed primary aggregation and secondary aggregation. In previous reports, we described that two macromolecules (aggregation factor [sAF] and aggregation receptor [AR] are involved in the secondary aggregation of sponge cells. The sAF was bound to a high-molecular-weight particle and was termed aggregation complex. The aggregation complex was shown to consist of two further functional subunits: UDP-glucuronosyltransferase and UDP-beta-D-galactosyltransferase. The AR with a molecular weight of approximately 17,000 was found to be a glycoprotein with D-glucuronic acid as the terminal sugar moiety. Data are presented from in vitro and in vivo experiments with the Geodia system, indicating that cell aggregation and cell separation are controlled first by alteration of the binding capacity of the aggregation receptor and second by an additional molecule (anti-aggregation receptor), which can decrease the interaction between the aggregation factor and the aggregation receptor. Recently we succeeded in the identification and isolation of the primary aggregation factor (pAF) from the same sponge species. This pAF is a glycoprotein that is firmly associated with the cell membrane. The Mr of the native pAF was 36,000; under denatured conditions three protein species were identified in the pAF preparation. We hypothesize that in contrast to the secondary aggregation, the initial aggregation of Geodia cells is mediated by the one-component system, the bivalent and bifunctional pAF. We were also able to dissociate the coral Eunicella cavolinii into single cells. These cells readily formed aggregates of a size of 2,100 micron during incubation in roller tubes: no aggregate formation was observed in non-rotating petri dishes. The formation of aggregates was not influenced by Ca++, urea or trypsin; it was also independent on temperature (4 degrees C to 30 degrees C) and pH (5.5-9.0). The intercellular material of the gorgonian contains a galactose-specific lectin, as determined by double diffusion experiments and haemagglutination inhibition experiments using a series of galacto-glycoconjugates. This lectin converted the aggregation-susceptible cells to aggregation-deficient cells.(ABSTRACT TRUNCATED AT 400 WORDS)