The N-terminal region of proto-dbl down regulates its transforming activity

The dbl proto-oncogene can transform NIH3T3 cells when overexpressed, but its transforming activity is about 50 to 70 fold lower than that of the dbl oncogene. The dbl oncogene encodes a protein of 478 amino-acids while proto-dbl encodes a protein of 925 amino-acids. The genesis of dbl involved the loss of the first 497 amino-acids of proto-dbl and the acquisition of a new N-terminus from another human locus. The last 428 amino-acids of proto-dbl and dbl product are identical with the exception of a single conservative amino-acid change. Any of these alterations could be responsible for the greater transforming activity of dbl. In order to define the role of these alterations more precisely, we constructed two deletion mutants, one derived from proto-dbl and the second from dbl in which only their last 428 amino-acids were retained. Under the control elements of the same promoter, the transforming activity of each of these mutants was similar to that of the dbl oncogene, i.e. 60-80 fold greater than that of proto-dbl. This finding suggests that the loss of the first 497 amino-acid of proto-dbl, rather than the acquisition of a new N-terminus, is crucial to the enhanced transforming activity of the dbl oncogene. Both mutant proteins were equally distributed between the membrane and cytosolic fractions, a pattern similar to that of their corresponding parental proteins. These results suggest that the subcellular distribution of proto-dbl is determined by its C-terminal 428 amino-acids. Unlike their parental proteins, neither mutant was phosphorylated, indicating that phosphorylation is not required for dbl transforming activity. In addition to the lack of phosphorylation, each mutant protein had a half-life of 5-6 h while the half-life of proto-dbl was about 1 h. Thus, our data suggest that the N-terminal half of proto-dbl can down regulate its transforming activity and that sequences within this region are responsible for rapid turnover of the protein.