Isolation and characterization of complementary DNA to proliferating cell nucleolar antigen P40

Proliferating cell nucleolar antigen P40 is a late G1-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 overlapping complementary DNA clones for antigen P40 were isolated by immunoscreening a lambda gt11 human expression library. The complete nucleotide sequence of the clones was determined. The complementary DNAs encode the Mr 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 G1 cells in cell cycle. A series of deletion mutants of the expressed peptide was constructed and the deletion mutants were expressed 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.