RETROVIRAL TRANSDUCTION AND ONCOGENIC SELECTION OF A CDNA-ENCODING DBS, A HOMOLOG OF THE DBL GUANINE-NUCLEOTIDE EXCHANGE FACTOR

A retroviral vector was used to transfer a large library of cDNAs from the 32D murine hemopoietic cell line to NIH3T3 fibroblasts, for the p urpose of Selecting cDNAs that induce oncogenic transformation. One hi ghly transformed colony arising in the infected NIH3T3 cell culture co ntained a provirus with a 1900 bp cDNA insert. After recovery and rein corporation into a retroviral vector, this cDNA induced rapid morpholo gical transformation add proliferation when expressed in NIH3T3 or C3H 10T1/2 fibroblast cell lines. The transforming cDNA encoded a protein, designated Dbs, which had a region of high sequence similarity to the Dbl proto-oncogene. This region included motifs characteristic of the CDC24 family of guanine nucleotide exchange factors, and an adjacent pleckstrin homology domain. Dbs was distinguished from Dbl by an N-ter minal extension and the presence of an SH3 domain at its C terminus. D eletions of the Dbs-encoding; cDNA demonstrated that transformation of NIH3T3 cells required intact exchange factor and pleckstrin homology domains, but dial not require the SH3 domain. In contrast to Dbl, the N-terminal sequences of Dbs did hot suppress its transforming activity . The Dbs gene was expressed at low levels in several murine hemopoiet ic cell lines and in thymus and spleen, and at higher levels in other tissues, particularly in brain. Dbs may be one of a large family of ex change factors which provide cell-type specific pathways for regulatin g proliferation via the activation of Ras-like proteins.