CLONING AND CHARACTERIZATION OF MN, A HUMAN TUMOR-ASSOCIATED PROTEIN WITH A DOMAIN HOMOLOGOUS TO CARBONIC-ANHYDRASE AND A PUTATIVE HELIX-LOOP-HELIX DNA-BINDING SEGMENT

MN is a transmembrane glycoprotein that has been detected in HeLa cell s and in some human carcinomas. The expression of MN protein in HeLa c ells is regulated by cell density. In HeLa x fibroblast cell. hybrids its expression correlates,vith tumorigenicity. Using a specific monocl onal antibody we have identified a cDNA clone coding for MN. Analysis of the deduced amino acid sequence revealed strong structural homology between the central region of the MN protein and carbonic anhydrases (CA). MN sequence retains the conserved zinc-binding site as well as t he enzyme's active center. In accord with these findings, MN protein f rom HeLa cells was found to bind zinc and to have carbonic anhydrase a ctivity. The N-terminal region of MN shares some similarity with DNA b inding proteins of the helix-loop-helix (HLH) family, and the protein was found to have affinity for DNA by DNA-cellulose chromatography. Th e region between the CA-like domain and the putative HLH domain is ric h in imperfect repeats of serine, proline, glycine and acidic residues with few hydrophobic amino acids, resembling thus an activation regio n of transcription factors. The fact that MN protein is detectable in several types of human carcinomas, but not in corresponding non-cancer ous tissues, suggests its possible role in neoplasia. In addition, the analysis of biological consequences of MN expression of NIH3T3 cells provides the evidence in favour of MN protein involvement in control o f cell proliferation and transformation.