Sharing of antigenic epitopes between synaptophysin and granulophysin

The immunological crossreactivity between the two granule‐specific membrane glycoproteins, synaptophysin and granulophysin, was studied using a series of site‐specific monoclonal and polyclonal antibodies. The epitope relatedness of six monoclonal antibodies against granulophysin was examined by competitive ELISA. The antibodies are shown to recognize distinct, but overlapping epitopes within a compact region that is constructed by the three‐dimensional configuration of the molecule. All these antibody clones also recognize rat neuronal synaptophysin. Two monoclonal antibodies against synaptophysin, of which one is the well‐characterized SY38 antibody, directed against the carboxy terminal of the molecule, are also shown to react with granulophysin. Characterized polyclonal antibodies against different peptide antigens of synaptophysin failed to recognize granulophysin. Synaptophysin and granulophysin are distinctly recognized in brain cell (white matter) and the pituitary both qualitatively and quantitatively. Based on these and other observations, it is suggested that the repeat motif in the cytoplasmic tail of synaptophysin represents an immunodominant construct that is the target for the observed crossreactive antibodies and that a similar tertiary construct has been preserved in granulophysin and in other transmembrane proteins.