The Role of Lectin I and of Glycoconjugates in Recognition of Cells from the Siliceous Sponge Geodia cydonium

Lectin I is very abundant in the extracellular matrix of the siliceous sponge Geodia cydonium. In the homologous system, it is involved in those processes which control the strength of the interaction between the aggregation factor and aggregation receptor. In the first part of the study, data on the binding affinity of the lectin to the cell surface, to the aggregation factor and to the glycoconjugates (extracellular matrix components) are given. Scatchard analyses revealed that the lectin binds to the cell surface with a K a of 4.2 × 10 6 M ‐1 , and to the aggregation factor with a K a of 35.4 × 10 6 M ‐1 . 2.6 × 10 5 lectin‐binding sites are present per sponge cell and 44 sites per aggregation factor. The lectin molecules were found to bind to each other with a K a of 0.9 × 10 6 M ‐1 . The receptor on the cell surface for this lectin was identified as the anti‐aggregation receptor, previously described. Purified glycoconjugates bind species‐specifically to the Geodia lectin. Functional studies revealed that the lectin alone has no aggregation promoting property; however, together with blood group A+L substance or with the aggregation factor the lectin allows or facilitates secondary aggregation. Furthermore, experimental evidence is presented to show that glycoconjugates (major matrix protein) are potential cell‐substrate adhesion molecules. Some parameters of this cell‐matrix interaction are given.