Identification of a novel Kruppel-associated box domain protein, Krim-1, that interacts with c-Myc and inhibits its oncogenic activity

We have used the Ras recruitment system to screen for proteins that interact with the N-terminally located transactivation domain of c-Myc. The Ras recruitment system is based on the activation of the mitogenic RAS signaling pathway in yeast by the mammalian GTPase Ha-Ras. This screen led to the identification of two novel nuclear proteins termed Krim-1A and Krim-1B that both contain an N-terminal KRAB box domain and 12 or 9 Kruppel C2H2 type zinc fingers at the C terminus, respectively. We found that sequences covering the Myc box II homology region are essential for the interaction with the Krim-1 proteins and that the second N-terminal zinc finger of Krim-1 is essential for Myc binding. Both Krim-1A and -B genes appear to be expressed ubiquitously with highest levels in spleen and lymph nodes. In particular, Krim-1B and, to a lesser extent, Krim-1A are able to decrease E-box-dependent transcriptional transactivation by c-Myc-Max complexes and also the ability of Myc to malignantly transform primary rat embryo fibroblasts, which is consistent with the functional repressive properties of their KRAB domains. The transcriptional corepressor Tif-1beta is a binding partner for Krim-1 and stabilizes the protein. Our findings suggest that Myc-mediated functions can be negatively regulated by Krim-1, potentially in a complex with Tif-1beta.