Biochemical genetics of blood group antigens: retrospect and prospect.

found in secretions and body fluids, and exhibit size heterogeneity upon purification, as discussed above. Based on these observations, it is likely that many glycosyltransferases share a common domain structure with the sialyltransferase. What is the basis for the subcellular localization of the glycosyltransferases? It has been demonstrated that the fi-galactoside a2,6-sialyltransferase is localized to the trans region of the Golgi apparatus stack in hepatocytes and in intestinal goblet cells, and is found throughout the Golgi apparatus of intestinal absorptive cells (Roth et al., 1985; 1986). Inspection of the sequence provides few clues as to the polypeptide domains which encode the information mediating subcellular localization. It is unlikely that the soluble catalytic domain found in secretions would determine localization. Probable candidates would include the cytoplasmic domain, the trans-membrane domain and the polypeptide sequences close to the lumenal side of the membrane. Now that a cDNA containing the complete coding sequence of the sialyltransferase has been obtained, it should be possible to carry out a systematic investigation of the molecular basis of the localization of the sialyltransferase within the Golgi apparatus of cells through the use of eukaryotic expression vectors and site-directed mutagenesis.