Proliferating Cell Nucleolar Antigen P40 Isolation and Characterization of Complementary DNA to

Proliferating cell nucleolar antigen P40 is a late Gi-specific protein, which was found in a variety of human tumors (A. Chatterjee, J. W. Freeman, and H. Busch. Cancer Res., 47:1123-1129, 1987). Two over lapping complementary DNA clones for antigen P40 were isolated by immunoscreening a Xgtll human expression library. The complete nucleotide sequence of the clones was determined. The complementary DNAs encode the M, 30,000 portion of the COOH-terminal portion of the protein. The mRNA for P40 was 2.8 kilobases long and was expressed maximally in Gìcells in cell cycle. A series of deletion mutants of the expressed peptide was constructed and the deletion mutants were ex pressed in Escherichia coli. Using these mutants, the epitope region of P40 recognized by a P40-specific monoclonal antibody was identified. The hydropathy plot based on the protein sequence revealed that this region of the protein is largely hydrophilic. This protein is unique and differs in sequence from other proliferating cell nuclear/nucleolar antigen proteins of similar molecular weight such as protein B23 and cyclin.