No abstract

CtBP has been shown to be a highly conserved corepressor of transcription. E1A and all the various transcription factors to which CtBP binds contain a conserved PLDLS CtBP-interacting domain, and EBNA3C includes a PLDLS motif (amino acids [aa] 728 to 732). Here we show that EBNA3C binds to CtBP both in vitro and in vivo and that the interaction requires an intact PLDLS. The C terminus of EBNA3C (aa 580 to 992) has modest trans-repressor activity when it is fused to the DNA-binding domain of Gal4, and deletion or mutation of the PLDLS sequence ablates this and unmasks a transactivation function within the fragment. However, loss of the CtBP interaction motif had little effect on the ability of full-length EBNA3C to repress transcription. A striking correlation between CtBP binding and the capacity of EBNA3C to cooperate with (Ha-)Ras in the immortalization and transformation of primary rat embryo fibroblasts was also revealed.CtBP (E1A C-terminal binding protein) was initially identified as a cellular factor interacting with the COOH terminus (amino acids [aa] 225 to 238) of adenovirus E1A oncoproteins. Although the precise significance of this interaction remains unknown, it is essential for the immortalization of primary rodent cells by E1A and has also been reported to negatively modulate E1A-mediated transformation, tumorigenicity, and metastasis (2,23,24,27,29). More recently it has been shown that this E1A-binding protein is one of a highly conserved family of (co)repressors of transcription. The Drosophila melanogaster homologue dCtBP mediates transcriptional repression by at least six different transcription factors, including Knirps, Kruppel, Snail, Polycomb, and Hairy (16,17,20,22,28). CtBP also interacts with Xenopus and human Polycomb proteins (28), and human hCtBP acts as a corepressor for the ZEB transcription factor that is involved in the regulation of lymphocyte and muscle differentiation (23). The mouse homologue mCtBP is a corepressor for the NET member of the Ets family of transcription factors and oncogenes (3). mCtBP also participates in the Ikaros repression complex (10). It has been reported that in some situations CtBP can recruit chromatin-modifying histone deacetylase (HDAC) enzymes 1, 4, 5, and 7 and that it can also bind Sin3A. However, the precise molecular mechanism by which CtBP inhibits transcription is unknown and may turn out to be different in different situations (3,10,15,30,36). All these various cellular transcription factors and also the E1A proteins contain a conserved Pro-X-Asp-Leu-Ser (PLDLS) CtBP interaction domain that is necessary and probably sufficient for the interaction. A second mammalian CtBP was recently described, and the two family members-which are referred to as CtBP1 and CtBP2-are largely homologous, although they may have distinct tissue distributions (5, 32). The protein described in this report is human CtBP1 and hereafter will be referred to as CtBP.The PLDLS amino acid motif is also found in a human cellular protein called CtIP (CtBP interacting prot...