A mitogenic lactose-binding lectin from the sponge Geodia cydonium

Area of Science:

• Biochemistry
• Immunology
• Marine Biology

Background:

• Lectins are proteins with carbohydrate-binding properties.
• Sponge lectins offer unique insights into immune interactions and molecular recognition.
• Hog A + H blood group substance is a complex glycoprotein antigen.

Purpose of the Study:

• To isolate and characterize lectins from the sponge Geodia cydonium with affinity for hog A + H blood group substance.
• To determine the carbohydrate-binding specificity and subunit structure of the purified lectin.
• To evaluate the mitogenic potential of the lectin on human lymphocytes.

Main Methods:

• Affinity chromatography using hog A + H coupled to Sepharose 4B for purification.
• Biochemical characterization including molecular weight, isoelectric point, and carbohydrate content analysis.
• Inhibition assays with various sugars and polysaccharides to determine binding specificity.
• Mitogenicity assays on human peripheral blood lymphocytes.

Main Results:

• Geodia lectin I was purified, revealing it as a glycoprotein (14% carbohydrate) with a molecular weight of 60,000 Da and an isoelectric point of pH 4.4.
• The lectin consists of disulfide-linked subunits of approximately 15,000 Da.
• Specificity was demonstrated for beta-linked D-galactose residues, with inhibition by lactose and specific disaccharides.
• Geodia lectin I exhibited mitogenic activity on human lymphocytes, with optimal concentration at 5.6 µg/ml in serum-free conditions.
• Fetal calf serum inhibited both hemagglutination and mitogenicity, suggesting specific binding interactions.

Conclusions:

• Geodia lectin I is a novel sponge lectin with a specific affinity for beta-linked D-galactose residues.
• The lectin possesses mitogenic properties for human lymphocytes, indicating potential immunomodulatory functions.
• The presence of inhibitory substances in fetal calf serum highlights the importance of the lectin's binding site in biological interactions.