Identification, purification and properties of a beta-1,3-glucan-specific lectin from the serum of the cockroach, Blaberus discoidalis which is implicated in immune defence reactions

A lectin specific for laminarin, a beta-1,3-glucan, agglutinating baker's y east and enhancing prophenoloxidase activation by laminarin, has been purif ied from the cockroach, Blaberus discoidalis, serum. Purification involved gel filtration with Bio-gel P300 and affinity chromatography on blue Sephar ose CL-6B and laminarin-Sepharose 4B. The purified lectin has a molecular m ass estimate of 520 kDa determined by gel filtration, and approximately 80 and 82 kDa by SDS-PAGE, under non-reducing and reducing conditions, respect ively. After isoelectric focusing the lectin focused as a single band at pH 4.9. The purified lectin was stained by the periodic acid/Schiffs reagent showing that it is a glycoprotein, and was deglycosylated by endo-beta-N-ac etylglucosaminidase F. Amino acid composition analysis showed the protein i s similar to previously purified beta-1,3-glucan binding proteins from othe r invertebrates. In electron micrographs by negative staining, the protein formed large aggregates with 'Y'-shaped 'structural units' ca. 79 x 65 nm. Immunological tests confirmed that this lectin is not related to any other lectins previously purified from the same insect. This protein appears to b e part of the hexamerin family of proteins. This is one of the first report s of a hexamerin-like molecule with lectin activity. (C) 1999 Elsevier Scie nce Inc. All rights reserved.