Molecular cloning and characterization of the human dbl proto-oncogene: evidence that its overexpression is sufficient to transform NIH/3T3 cells.

We isolated cDNA clones representing the human dbl proto-oncogene transcript. Nucleotide sequence analysis revealed an open reading frame encoding a predicted protein of 925 amino acids. Using peptide antisera directed against specific proto-dbl peptides, a 115-kd protein was detected in COS cells transfected with an expression vector containing the entire coding region of proto-dbl. This mol. wt is consistent with that predicted from the open reading frame. We have previously shown that the dbl oncogene was generated by substitution of the 5' portion of proto-dbl with an unrelated human sequence. In this study we show that this rearrangement resulted in the loss of the 497 amino-terminal codons of the dbl proto-oncogene. Under the influence of a strong promoter proto-dbl could readily transform NIH/3T3 cells but its transforming activity was less than that of the dbl oncogene driven by the same promoter. Proto-dbl overexpression is, therefore, sufficient to transform NIH/3T3 cells, but specific structural alterations of its coding region significantly enhance its transforming activity. No apparent similarity was detected between the predicted proto-dbl product and other known proto-oncogenes. However, a stretch of 300 amino acids within the N-terminal half of proto-dbl showed structural similarity to the intermediate filament vimentin. This region in proto-dbl contains a heptad repeat motif characteristic of an alpha-helical coiled-coil structure. Taken together, these findings indicate that the human proto-dbl represents a new class of cellular oncogenes that may be related to cytoskeletal elements of the cell.