TOJ3, a target of the v-jun transcription factor, encodes a protein with transforming activity related to human microspherule protein 1 (MCRS1)

Using the established quail cell line Q/d3 conditionally transformed by the v-jun oncogene, cDNA clones (TOJ2, TOJ3, TOJ5, TOJ6) were isolated by repr esentational difference analysis (RDA) that correspond to genes which were induced immediately upon conditional activation of v-jun. One of these gene s, TOJ3, is immediately and specifically activated after doxycycline-mediat ed v-jun induction, with kinetics similar to the induction of well characte rized direct AP-I target genes. TOJ3 is neither activated upon conditional activation of v-myc, nor in cells or cell lines non-conditionally transform ed by oncogenes other than v-jun. Sequence analysis revealed that the TOJ3- specific cDNA encodes a 530-amino acid protein with significant sequence si milarities to the murine or human microspherule protein 1 (MCRS1, MSP58), a nucleolar protein that directly interacts with the ICP22 regulatory protei n from herpes simplex virus 1 or with p120, a proliferation-related protein expressed at high levels in most human malignant tumor cells. Similar to i ts mammalian counterparts, the TOJ3 protein contains a bipartite nuclear lo calization motif and a forkhead associated domain (FHA). Using polyclonal a ntibodies directed against a recombinant amino-terminal TOJ3 protein segmen t, the activation of TOJ3 in jun-transformed fibroblasts was also demonstra ted at the protein level by specific detection of a polypeptide with an app arent M-r of 65000. Retroviral expression of the TOJ3 gene in quail or chic ken embryo fibroblasts induces anchor age-independent growth, indicating th at the immediate activation of TOJ3 in fibroblasts transformed by the v-jun oncogene contributes to cell transformation.