Abstract
beta-1,4-Galactosyltransferase (GalTase) is localized to two subcellular compartments, the Golgi complex, where it participates in cellular glycosylation, and the plasma membrane, where it functions as a receptor for oligosaccharide ligands on opposing cells or in the extracellular matrix. The gene for GalTase encodes two nearly identical proteins that differ only in their N-terminal cytoplasmic domains: both short and long GalTases share an 11-aa cytoplasmic tail, but long GalTase has an additional 13-aa sequence on its cytoplasmic domain. In this study, we investigated the subcellular distribution of endogenous long GalTase in untransfected F9 and 3T3 cells by using confocal microscopy and antibodies specific for the 13-aa sequence unique to long GalTase. Long GalTase was found in the Golgi complex as expected; long GalTase was also found on the plasma membrane in cell-type-specific distributions. In 3T3 cells, long GalTase was evident on the basal surface of cells possessing a migratory phenotype, being concentrated at the leading and trailing edges; nonmigratory cells had little detectable surface immunoreactivity. In F9 cells, long GalTase was localized on the plasma membrane, being concentrated at the apical aspect of intercellular junctions. These results demonstrate that in 3T3 and F9 cells, long GalTase is present on the cell surface in addition to the Golgi complex. The pattern of surface expression shows cell-type specificity that is consistent with GalTase function in cellular interactions.