Species-specific aggregation factor in sponges. VI. Aggregation receptor from the cell surface.

An aggregation receptor from the siliceous sponge Geodia cydonium has been isolated and purified in an almost pure form. It sediments at about 2-6s, has a buoyant density of 1-51 g/ml in CsCl and elutes from Sephadex G-50 at a Ve/V0 value of 1-311. Chemical analysis revealed that the receptor consists of 81% neutral carbohydrate and 7-5% protein. The activity of the receptor is rapidly destroyed by Na-periodate. The receptor is released from the cell surface after removal of Ca2+ from the medium or after incubation of the cells with trypsin. The depleted cells can be charged again with isolated receptor molecules. The binding of the receptor molecules on the cell surface is prevented in the presence of trypsin. For optimal binding, physiological salt concentrations with respect to NaCl (540 mM NaCl) and Ca2+ ions are necessary. The receptor whose isolation is described in this report, is involved in secondary aggregation processes, which are initiated by a soluble aggregation factor. The primary aggregation of the cells is not influenced by the receptor. Time-course studies with receptor-depleted cells revealed that new aggregation receptor molecules are formed during the aggregation process. By competition experiments it could be shown that high concentrations of soluble aggregation receptor molecules inhibit secondary aggregation. The soluble receptor molecules can complete with surface-bound receptor molecules only if these are not linked with the aggregation factor.