Biochemical and immunological comparisons of carbohydrate-binding protein 35 and an IgE-binding protein

The predicted amino acid sequence of carbohydrate-binding protein 35 (CBP35; Mr approximately 35,000), a galactose-specific lectin in many mouse and human cells, has been compared to the predicted sequence of an IgE-binding protein (epsilon BP) originally identified in rat basophilic leukemia cells. The sequences of the two proteins showed that: (a) 85% of the amino acid residues are identical; (b) the polypeptide chains are comprised of two distinct domains; and (c) highly conserved internal repetitive sequences are present. Consistent with these observations, antisera raised against CBP35 or against a synthetic peptide derived from the epsilon BP sequence cross-reacted with both proteins. Moreover, fractionation of extracts of mouse 3T3 fibroblasts over an IgE-Sepharose affinity column showed that CBP35 bound to IgE; this binding was reversed by addition of lactose. Conversely, fractionation of extracts of rat basophilic leukemia cells over an affinity column of Sepharose derivatized with N-(epsilon-amino-caproyl)-D-galactosamine showed that epsilon BP was a galactose-binding protein. Furthermore, epsilon BP bound to IgE-Sepharose could be eluted by lactose. Finally, transcription and translation in vitro of the cDNA corresponding to epsilon BP yielded a polypeptide containing carbohydrate-binding activity. All of these results strongly support the conclusion that CBP35 and epsilon BP are mouse and rat homologues, respectively.