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

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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 .times. 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 MW from 44,000 to 56,000 (electrophoretic determination) or from 36,500 to 53,500 (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 3-dimensional clumps (diameter up to 8 .mu.m). Turbidimetric studies reveal an inhibition of the lectin polymerization by lactose. The Ka of the lectin-lectin polymerization rises from 0.9 .times. 106 M-1 to 14.0 .times. 106 M-1 after increasing the Ca2+ concentration (from 1 .mu.M to 100 .mu.M). Parallel with this increase in affinity, the Ka value of the lectin-aggregation factor binding drops from 41.2 .times. 106 M-1 (1 .mu.M Ca2+) to 1.3 .times. 106 M-1 (100 .mu.M Ca2+). In the absence of Ca2+, the Geodia lectin forms 1-10-.mu.m 2-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 (L5178y) bind with a higher affinity to the lectin-polymer matrix than to the lectin-monomer one. Lectin-polymer structures, together with lectin-glycoconjugate associates, are probably components of the cell-substrate adhesion system(s) of sponges in vivo.