EVIDENCE THAT CELL-SURFACE BETA-1,4-GALACTOSYLTRANSFERASE SPONTANEOUSLY GALACTOSYLATES AN UNDERLYING LAMININ SUBSTRATE DURING FIBROBLAST MIGRATION

beta 1,4-Galactosyltransferase is unusual among the glycosyltransferas es in that a subpopulation exists on the cell surface in addition to i ts traditional biosynthetic location within the Golgi complex, On the cell surface, galactosyltransferase is expressed in spatially restrict ed, cell type-specific domains, where it functions as a receptor for e xtracellular oligosaccharide ligands during selected cellular interact ions, For example, galactosyltransferase is found on the leading and t railing edges of migrating cells, where it facilitates lamellipodia fo rmation and cell spreading by binding to specific N-linked oligosaccha rides within laminin. Although the ability of galactosyltransferase to serve as a laminin receptor is well documented, it is unclear whether it functions solely in a lectin-like capacity to bind laminin glycosi de ligands or uses its intrinsic catalytic activity to release itself from and modify its oligosaccharide substrate. In this study, we deter mined whether cell surface galactosyltransferase spontaneously galacto sylates laminin matrices during cell migration using endogenous galact ose donors, Cells were prelabeled with [H-3]galactose, washed, and tra nsferred in small clusters onto laminin matrices, The prelabeled cells migrated out from the cell cluster, during which time they deposited covalently bound [H-3]galactose residues onto the laminin matrix. The degree of galactosylation was both laminin- and time-dependent and req uired actively migrating intact cells. The radioactivity released from the H-3-galactosylated laminin by acid hydrolysis comigrated with aut hentic galactose standards on paper chromatography. In parallel assays , there was no radioactivity deposited on laminin matrices when cells were prelabeled with [H-3]fucose or [H-3]leucine, Furthermore, [H-3]ga lactosylation was dependent upon galactosyltransferase-mediated cell m igration, since prelabeled cells did not deposit [H-3]galactose when m igrating on fibronectin, upon which migration is integrin-dependent an d galactosyltransferase-independent. These results raise the possibili ty that galactosyltransferase functions catalytically during cell migr ation, either to dissociate from its oligosaccharide Ligand and/or to modify the extracellular matrix.