Physicochemical and functional characterization of the polymerization process of the Geodia cydonium lectin

Abstract

The extracellularly localized, galactose‐specific lectin from the sponge Geodia cydonium binds at one class of sites, 40 mol Ca2+/mol lectin with an association constant (Ka) of 0.3 × 106 M−1. Stoichiometric calculations reveal that in the extracellular milieu 22 mol Ca2+ (maximum) are complexed per mol lectin. Binding of Ca2‐ to the lectin increases its apparent Mr from 44000 to 56000 (electrophoretic determination) or from 36500 to 53500 (high‐pressure liquid gel chromatographical determination); the S20,w increases from 4.3 S to 4.5 S if Ca2‐ is added to the lectin. In the presence of Ca2+ the lectin undergoes a conformational change perhaps by expanding the carbohydrate side chains which are terminated by galactose. Subsequently the lectin molecules polymerize to large three‐dimensional clumps (diameter up to 8 μm). Turbidimetric studies reveal an inhibition of the lectin polymerization by lactose. The Ka of the lectin‐lectin polymerization rises from 0.9 × 106 M−1 to 14.0 × 106 M−1 after increasing the Ca2+ concentration (from 1 μM to 100 μM). Parallel with this increase in affinity, the Ka value of the lectin‐aggregation factor binding drops from 41.2 × 106 M−1 (1 μM Ca2+) to 1.3 × 106 M−1 (100 μM Ca2+). In the absence of Ca2+, the Geodia lectin forms 1–10‐μm two‐dimensional sheets in the presence of homologous glycoconjugates. Cell binding experiments with polyacrylamide gels, containing covalently bound galactose, show that both homologous (Geodia cydonium) and heterologous cells (L 5178y) bind with a higher affinity to the lectin‐polymer matrix than to the lectin‐monomer one. These data suggest that lectin‐polymer structures, together with lectin‐glycoconjugate associates, are components of the cell‐substrate adhesion system(s) of sponges in vivo. Copyright © 1985, Wiley Blackwell. All rights reserved