Recent data from this laboratory suggests that an IgA receptor, secretory component (SC) is a galactosyltransferase (GT). In the presence of UDPgalactose, the enzyme transfers galactose to IgA. In the absence of the sugar donor, however, the enzyme merely binds to the IgA. The goal of this research is to determine whether secretory component is a galactosyltrasnferase by cloning and sequencing the gene that encodes the IgA-specific galactosyl- transferase. To isolate the gene, antibodies will be used to identify bacteria transfected with GT cDNA. In addition, a novel, functional screen will be employed. This screen is based on four, linked enzymes that produce insoluble formazan precipitates over cells that catalyze the transfer of galactose to a particular acceptor--in this case, IgA. A functional screen is critical because immunological probes might not discriminate among structurally related transferases, or transferase fragments with shared epitopes and similar molecular weights. Recent, preliminary biochemical, immunological, and functional evidence suggests that a natually occurring, antibody binding protein, secretory component (which is, itself, a primitive member of the immunoglobulin superfamily of proteins) may be an enzyme that not only recognizes and binds to the antibody (IgA) with which it associates, but also attaches sugar molecules to the IgA. If the critical test proposed here demonstrates clearly that secretory component is, indeed, a sugar transfering enzyme, this would be the first clear case in which an extracellular receptor recognizes its ligand and modifies it chemically by the addition of sugars. The demonstration of this process would imply a new mechanism by which receptor-ligand interactions are established and modified and would have important implications for our understanding of how signals are transmitted between cells.
