Human IgE-binding protein: a soluble lectin exhibiting a highly conserved interspecies sequence and differential recognition of IgE glycoforms

Abstract

IgE-binding protein (epsilon BP) refers to a protein originally identified in rat basophilic leukemia cells by virtue of its affinity for IgE. It is now known to be a beta-galactoside-binding lectin equivalent to carbohydrate-binding protein 35 (CBP 35). More recently, its identity to Mac-2, a macrophage cell-surface protein, has been established. cDNA coding for human epsilon BP has been cloned from a human HeLa cell cDNA library and contains an open reading frame of 750 base pairs encoding a 250 amino acid protein. Like the rat and murine counterparts, the human epsilon BP amino acid sequence can be divided into two domains with the amino-terminal domain consisting of a highly conserved repetitive sequence (YPGXXXPGA) and the carboxyl-terminal domain containing sequences shared by other S-type lectins. The human epsilon BP sequence exhibits extensive homology to murine and rat epsilon BP with 84% and 82% identity, respectively. The homology is particularly striking in the carboxyl-terminal domain where 95% identity is found between human and murine sequences in a stretch of over 70 amino acids. A survey of epsilon BP mRNA expression from several lymphocyte cell lines revealed that the level of epsilon BP transcription may reflect a relationship between cell differentiation and epsilon BP expression. Finally, human epsilon BP was purified from several human cell lines and shown to possess lactose-binding characteristics and cross-species reactivity to murine IgE. Surprisingly, three different human myeloma IgE proteins did not show reactivity to human epsilon BP. However, after neuraminidase treatment of each human IgE, pronounced binding to epsilon BP was observed, thereby indicating that only specific IgE glycoforms can be recognized by epsilon BP.