Proteins encoded by the proto‐oncogenes c‐myc, L‐myc, and N‐myc contain at their carboxy‐terminus a tripartite segment comprising a basic DNA binding region (BR), a helix‐loop‐helix (HLH) and a leucine zipper motif (Zip), that are believed to be involved in DNA binding and protein‐protein interaction. The N‐Myc oncoprotein is overexpressed in certain human tumors that share neuroectodermal features due to amplification of the N‐myc gene. Using a monoclonal antibody directed against an N‐terminal epitope of the N‐Myc protein in immunoprecipitations performed with extracts of neuroblastoma cells, two nuclear phosphoprotein, p20/22, forming a hetero‐oligomeric complex with N‐Myc are identified. Both proteins are phosphorylated by casein kinase II in vitro. By partial proteolytic maps we show that p20 and p22 are structurally related to each other and that p20 is identical with Max, a recently described in vitro binding partner of myc proteins. Time course experiments show the presence of the complex in cellular extracts immunoprecipitated within a 5 min interval after the preparation of the cell extract. While the expression of N‐myc is restricted, expression of both Max(p20/22) and the murine homolog Myn(p20/22) was observed in cells of diverse human and murine embryonal lineages as detected by heterologous complex formation. By introduction of expression vectors containing the wild type N‐myc gene or N‐myc genes with in frame deletions or point mutations into recipient cells and subsequent immunoprecipitation of the resulting N‐Myc proteins we show that the HLH‐Zip region is essential to the formation of the N‐Myc‐p20/22 complex.

Human neuroblastoma cells frequently show amplification of the oncogene MYCN, which maps to 2p24. Previous studies have localized the DEAD box motif gene DDX1 to the same chromosome band and demonstrated coamplification of DDX1 and MYCN in two retinoblastoma cell lines. Recently, a high frequency of coamplification of DDX1 and MYCN has been shown in human neuroblastoma cells. We have determined the physical distance between the two genes by pulsed field gel electrophoresis in normal tissue and have found that DDX1 maps to a position at a maximum distance of 400 kbp 5' to MYCN. Two neuroblastoma cell lines with coamplification of DDX1/MYCN showed a similar topographic relationship of the two genes. In contrast, in two cell lines with high copy number, the DDX1 gene was not present in all amplified units recognized by MYCN and had changed its position in the amplified DNA relative to MYCN from 5' to 3', presumably by rearrangement during the amplification process. Our data show that the high frequency of DDX1 coamplification is due to its close physical distance to MYCN. Although amplification has resulted in an elevated expression of DDX1 the significance of overexpression for neuroblastoma remains unclear.

No abstract