Identification of an Epitope Region of the Human Proliferation-associated Nucleolar Antigen P1201

An epitope region, located at amino acid residues 173-180 (EAAAGIQW), of a human cell proliferation-associated nucleolar antigen, PI 20, has been defined by mutational analysis and competition assays. A synthetic peptide corresponding to this epitope region completely blocks the binding of the anti-Pi 20 antibody to Escherichia co/i-expressed PI 20 and the HeLa nucleolar PI20 protein. Adjacent peptides lack inhibitory effects. The antigenic site includes a hydrophilic residue and a hydrophobic stretch. The glutamyl and tryptophanyl residues in this region make major contributions to the binding of PI 20 to its antibody, since peptides lacking either the glutamyl or the tryptophanyl residue do not block the antibody binding to the PI20 antigen. This study provides a basis for drug design for specific binding to the epitope region of the PI 20 protein.