We used a yeast two-hybrid screening approach to identify novel interactors of CCAAT/enhancer-binding protein ␣ (C/EBP␣) that may offer insight into its mechanism of action and regulation. One clone obtained was that for CA150, a nuclear protein previously characterized as a transcriptional elongation factor. In this report, we show that CA150 is a widely expressed co-repressor of C/EBP proteins. Two-hybrid and co-immunoprecipitation analyses indicated that CA150 interacts with C/EBP␣.

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... ssion of CA150 inhibited the transactivation produced by C/EBP␣ and was also able to reverse the enhancing effect of the co-activator p300 on C/EBP␤mediated transactivation. Analysis of C/EBP␣ mutants indicated that CA150 interacts with C/EBP␣ primarily through a domain spanning amino acids 135-150. Chromatin immunoprecipitation assays showed that CA150 was present on a promoter that is repressed by C/EBP␣ but not present on a promoter that is activated by C/EBP␣. Finally, we showed that in cells in which growth arrest had been induced by ectopic expression of C/EBP␣, CA150 was able to release them from growth arrest. Interestingly, CA150 could not reverse the growth arrest produced by the minimal growth-arrest domain of C/EBP␣ (amino acids 175-217), suggesting that the effect of CA150 was directed at a region of C/EBP␣ outside of this minimal domain, consistent with our two-hybrid analysis. Taken together, these data indicate that CA150 is a co-repressor of C/EBP proteins and provides a possible mechanism for how C/EBP␣ can repress transcription of specific genes.