Evidence for a molecular distinction between Golgi and cell surface forms of beta 1,4-galactosyltransferase.

beta 1,4-Galactosyltransferase (GalTase) is present on the plasma membrane of many cell types in addition to its traditional location within the Golgi compartment. Recently, the GalTase gene has been shown to encode two proteins that are identical throughout their length except that one has an additional 13-amino acid extension in its amino-terminal cytoplasmic domain. We present evidence here suggesting that the longer GalTase protein, containing this unique 13-amino acid peptide, is preferentially targeted to the plasma membrane, and the shorter GalTase protein resides primarily within the Golgi compartment. S1 nuclease protection assays of RNA from a variety of cells and tissues show that the relative abundance of the short and long GalTase mRNAs correlates with GalTase-specific activities in the Golgi and plasma membranes, respectively. Furthermore, transfection of cDNAs encoding either the long or short GalTase protein into F9 embryonal carcinoma cells suggests that the long GalTase protein is preferentially expressed on the cell surface. These results propose a molecular distinction between the Golgi and cell surface forms of GalTase as well as a novel mechanism for targeting glycoproteins to the cell surface.