Cell-cell recognition: specific binding of Microciona sponge aggregation factor to homotypic cells and the role of calcium ions

Previous studies have established that the mechanism of species-specific sponge cell reaggregation in vitro involves at least three components: a species-specific aggregation factor, its cell surface receptors, and Ca2+ ions. Here we report about the binding of Microciona 125I-labeled aggregation factor to cells and to other molecules of aggregation factor coupled to agarose beads, as well as the role of divalent cations in both factor-cell and factor-factor interactions. Our results indicate that cell aggregation is a two-step process, involving (1) Ca2+-independent species-specific binding of the 21 X 106 dalton aggregation factor to cellular receptors and (2) formation of Ca2+-dependent linkages between factor molecules on adjacent cells. Quantitation of binding suggests that less than 5% of the cell-associated aggregation factor in vivo is required for cell-cell adhesion in the in vitro assay. Alternative biological functions for the aggregation factor are discussed.