Biochemical characterization of the mammalian Cux2 protein

The CCAAT displacement protein (CDP) and cux (Cut homeobox) genes were originally identified as the human and mouse orthologs of Drosophila melanogaster cut. More recently, vertebrates were found to possess a second cut orthologs that was generated by gene duplication: Cux2. We report the initial biochemical characterization of the Cux2 protein in tissue culture and in vitro. We generated four polyclonal antibodies that were able to recognize the human and mouse Cux2 protein but displayed little or no cross-reactivity towards CDP1 and Cux1. The expression of the Cux2 protein was convincingly detected in only one among 19 neuronal cell lines: the SH-SY5Y human neuroblastoma cell line. CDP/Cux proteins contain four DNA binding domains, three Cut repeat (CR1, CR2 and CR3) and one Cut homeodomain (HD). Purified fusion proteins containing either CR1CR2, CR2CR3HD or CR3HD exhibited similar DNA binding specificities as the corresponding domains of Cux1, but their DNA binding kinetics were much more rapid. Similarly, the full-length Cux2 protein made rapid but transient interactions with DNA. We did not observe an N-terminally processed Cux2 isoform equivalent to the Cux1 p110 isoform. Whereas Cux1 can function as a repressor or activator in a promoter-specific manner, Cux2 functioned exclusively as a transcriptional repressor in NIH3T3 cells. Overall, our results suggest that the Cux1 and Cux2 proteins carry distinct biochemical functions. Cux2 is able, like Cux1, to perform the CCAAT-displacement activity. However, Cux2 is unlikely to execute transcriptional regulatory functions that require stable interaction with DNA.