COMPLEMENTARY ACCEPTOR AND SITE SPECIFICITIES OF FUC-TIV AND FUC-TVIIALLOW EFFECTIVE BIOSYNTHESIS OF SIALYL-TRILEX AND RELATED POLYLACTOSAMINES PRESENT ON GLYCOPROTEIN COUNTERRECEPTORS OF SELECTINS

The P-selectin counterreceptor PSGL-1 is covalently modified by mono a lpha 2,3-sialylated, multiply alpha 1,3-fucosylated polylactosamines. These glycans are required for the adhesive interactions that allow th is adhesion receptor-counterreceptor pair to facilitate leukocyte extr avasation, To begin to understand the biosynthesis of these glycans, w e have characterized the acceptor and site specificities of the two gr anulocyte alpha 1,3-fucosyltransferases, Fuc-TIV and Fuc-TVII, using r ecombinant forms of these two enzymes and a panel of synthetic polylac tosamine-based accepters. We find that Fuc-TIV can transfer fucose eff ectively to all N-acetyllactosamine (LN) units in neutral polylactosam ines, and to the ''inner'' LN units of alpha 2,3-sialylated accepters but is ineffective in transfer to the distal alpha 2,3-sialylated LN u nit in alpha 2,3-sialylated accepters, Fuc-TVII, by contrast, effectiv ely fucosylates only the distal alpha 2,3-sialylated LN unit in alpha 2,3-sialylated accepters and thus exhibits an acceptor site-specificit y that is complementary to Fuc-TIV, Furthermore, the consecutive actio n of Fuc-TIV and Fuc-TVII, in vitro, can convert the long chairs sialo glycan SA alpha 2-3'LN beta 1-3'LN beta 1-3'LN (where SA is sialic aci d) into the trifucosylated molecule SA alpha 2-3'Lex beta 1-3'Lex beta 1-3'Lex (where Lex is the trisaccharide Gal beta 1-4(Fuc alpha 1-3)Gl cNAc) known to decorate PSGL-1, The complementary in vitro acceptor si te-specificities of Fuc-TIV and Fuc-TVII imply that these enzymes coop erate in vivo in the biosynthesis of monosialylated, multifucosylated polylactosamine components of selectin counterreceptors on human leuko cytes.