Complementary acceptor and site specificities of Fuc-TIV and Fuc-TVII allow effective biosynthesis of sialyl-TriLex and related polylactosamines present on glycoprotein counterreceptors of selectins

Abstract

The P-selectin counterreceptor PSGL-1 is covalently modified by mono α2,3-sialylated, multiply α1,3-fucosylated polylactosamines. These glycans are required for the adhesive interactions that allow this adhesion receptor- counterreceptor pair to facilitate leukocyte extravasation. To begin to understand the biosynthesis of these glycans, we have characterized the acceptor and site specificities of the two granulocyte α1,3- fucosyltransferases, Fuc-TIV and Fuc-TVII, using recombinant forms of these two enzymes and a panel of synthetic polylactosamine-based acceptors. We find that Fuc-TIV can transfer fucose effectively to all N-acetyllactosamine (LN) units in neutral polylactosamines, and to the 'inner' LN units of α2,3- sialylated acceptors but is ineffective in transfer to the distal α2,3- sialylated LN unit in α,3-sialylated acceptors. Fuc-TVII, by contrast, effectively fucosylates only the distal α2,3-sialylated LN unit in α2,3- sialylated acceptors and thus exhibits an acceptor site-specificity that is complementary to FucTIV. Furthermore, the consecutive action of Fuc-TIV and Fuc-TVII, in vitro, can convert the long chain sialoglycan SAα2-3'LNβ1- 3'LNβ1-3'LN (where SA is sialic acid) into the trifucosylated molecule SAα2-3'Lexβ1-3'Lexβ1-3'Lex (where Lex is the trisaccharide Galβ1- 4(Fucα1-3)GlcNAc) known to decorate PSGL-1. The complementary in vitro acceptor site-specificities of FucTIV and Fuc-TVII imply that these enzymes cooperate in vivo in the biosynthesis of monosialylated, multifucosylated polylactosamine components of selectin counterreceptors on human leukocytes.