BIOSYNTHESIS OF BRANCHED POLYLACTOSAMINOGLYCANS - EMBRYONAL CARCINOMA-CELLS EXPRESS MIDCHAIN BETA-1,6-N-ACETYLGLUCOSAMINYLTRANSFERASE ACTIVITY THAT GENERATES BRANCHES TO PREFORMED LINEAR BACKBONES

Two types of beta 1,6-GlcNAc transferases (IGnT6) are involved in in v itro branching of polylactosamines: dIGnT6 (distally acting), transfer ring to the penultimate galactose residue in accepters like GlcNAc bet a 1-3Gal beta 1-4GlcNAc beta 1- R, and cIGnT6 (centrally acting), tran sferring to the midchain galactoses in accepters of the type (GlcNAc b eta 1-3)Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-R. The ro les of the two transferases in the biosynthesis of branched polylactos amine backbones have not been clearly elucidated. We report here that cIGnT6 activity is expressed in human (PA1) and murine (PC13) embryona l carcinoma (EC) cells, both of which contain branched polylactosamine s in large amounts. In the presence of exogenous UDP-GlcNAc, lysates f rom both EC cells catalyzed the formation of the branched pentasacchar ide Gal beta 1-4GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc fro m the linear tetrasaccharide Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4Gl cNAc. The PA1 cell lysates were shown to also catalyze the formation o f the branched heptasaccharides Gal beta 1-4G1cNAc beta 1-3Gal beta 1- 4GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc and Gal beta 1-4Gl cNAc-beta 1-3(GlcNAc beta 1-6) Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4 GlcNAc from the linear hexasaccharide Galpl-LiGlcNAcpl3Galpl-4GlcNAcpl 3GalP1-4GlcNAc in reactions characteristic to cIGnT6, By contrast, dIG nT6 activity was not detected in the lysates of the two EC cells that were incubated with UDP-GlcNAc and the acceptor trisaccharide GlcNAc b eta 1-3Gal beta 1-4GlcNAc. Hence, it appears likely that cIGnT6, rathe r than dIGnT6 is responsible for the synthesis of the branched polylac tosamine chains in these cells.