A human cDNA corresponding to a gene overexpressed during cell proliferation encodes a product sharing homology with amoebic and bacterial proteins

Abstract

A clone, designated pag, was isolated by differential screening of cDNA libraries made from the untransformed and ras-transformed human mammary epithelial cell line HBL100. This cDNA corresponds to a gene constitutively expressed in most human cells which is induced to higher levels upon serum stimulation in untransformed and ras-transformed HBL100 cells. However, the abundance of the pag transcript is approximately 3-fold higher in transformed as compared to untransformed cells after 7-15 h of serum stimulation. In the promyelocytic leukemia cell line HL60 induced to differentiate the level of pag mRNA starts to decrease between 48 and 72 h following induction. During this period, which represents the commitment phase of differentiation, HL60 cells cease to proliferate. Therefore, in HBL100 and HL60 cells, higher levels of pag gene expression are correlated with cell proliferation. The pag cDNA codes for a 22-kDa protein, devoid of known consensus motifs, and shares 66% homology with a murine gene product (MER5) that is preferentially expressed in erythroleukemia cells during the early period of cell differentiation. In addition, the pag gene product shares approximately 50% identity with a 29-kDa surface antigen of Entamoeba histolytica and a 26-kDa antigen of Helicobacter pylori. Distant relationship was also found with other prokaryotic proteins. The pag cDNA hybridizes to multiple sequences within human and other mammalian genomes and to fewer sequences in chicken and Saccharomyces cerevisiae. Although a true relationship between eukaryotic and prokaryotic genes is difficult to establish, the conservation of pag gene sequences throughout Eukaryotae rather suggests that the pag locus belongs to a new class of genes encoding highly conserved proteins.