Human Golgi β‐galactoside α‐2,6‐sialyltransferase generates a group of sialylated B lymphocyte differentiation antigens

The role of the human β‐galactoside α‐2,6‐sialyltransferase (hu α‐2,6‐ST) in the generation of B cell surface antigens was investigated by selecting subclones of COS cells (monkey kidney epithelial cells) constitutively expressing a transfected cDNA which encodes the hu α‐2,6‐ST (COS α‐2,6‐ST cells). Expression of hu α‐2,6‐ST in COS cells was sufficient to generate sialylated cell surface epitopes on different glycosylated antigens recognized by monoclonal antibodies to CDw75, CD76, and the unclustered monoclonal antibodies HB‐4 and EBU‐65. These epitopes were sensitive to sialidase treatment and are likely to contain terminal α‐2,6‐linked sialic acid residues. A novel antiserum raised against bacterially expessed hu α‐2,6‐ST fusion protein was used to localize the sialyltransferase in two cell lines with high expression of either endogenous (B cell line JOK‐1) or recombinant (COS α‐2,6‐ST cells) hu α‐2,6‐ST. In both cell lines, the enzyme was detected only intracellularly in the juxtanuclear region and not on the cell surface. In contrast, CDw75, formerly proposed to be identical with an α‐2,6‐ectosialyltransferase, was strongly expressed on the cell surface. The different expression patterns show that neither the CDw75 antigen nor any of the other sialylated antigens analyzed is identical with the hu α‐2,6‐ST. Furthermore, the presence of a surface‐expressed α‐2,6‐ST appears unlikely in these cell lines. We propose that CDw75, CD76, HB‐4, and EBU‐65 represent a unique group of B cell differentiation antigens the production of which requires the enzymatic activity of α‐2,6‐ST.