LOCALIZATION OF THE LONG FORM OF BETA-1,4-GALACTOSYLTRANSFERASE TO THE PLASMA-MEMBRANE AND GOLGI-COMPLEX OF 3T3 AND F9 CELLS BY IMMUNOFLUORESCENCE CONFOCAL MICROSCOPY

beta-1,4-Galactosyltransferase (GalTase) is localized to two subcellul ar compartments, the Golgi complex, where it participates in cellular glycosylation, and the plasma membrane, where it functions as a recept or for oligosaccharide ligands on opposing cells or in the extracellul ar matrix. The gene for GalTase encodes two nearly identical proteins that differ only in their N-terminal cytoplasmic domains: both short a nd 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 GalT ase in untransfected F9 and 3T3 cells by using confocal microscopy and antibodies specific for the 13-aa sequence unique to long GalTase. Lo ng GalTase was found in the Golgi complex as expected; long GalTase wa s also found on the plasma membrane in cell-type-specific distribution s. In 3T3 cells, long GalTase was evident on the basal surface of cell s possessing a migratory phenotype, being concentrated at the leading and trailing edges; nonmigratory cells had little detectable surface i mmunoreactivity. In F9 cells, long GalTase was localized on the plasma membrane, being concentrated at the apical aspect of intercellular ju nctions. These results demonstrate that in 3T3 and F9 cells, long GalT ase is present on the cell surface in addition to the Golgi complex. T he pattern of surface expression shows cell-type specificity that is c onsistent with GalTase function in cellular interactions.