Cux/CDP homeodomain protein binds to an enhancer in the rat c-mos locus and represses its activity

The c-mos gene is transcribed in male and female germ cells, in differentiating myoblasts and in 3T3 cells from cell-specific promoters. We characterized the rat testis promoter, which contains a TATA-box and one binding site for a testis-specific transcription factor TTF-D, as well as a region which can act as enhancer, which is located approx. 2 kb upstream of the c-mos AUG start codon. It binds three factors at sites I, II and III as determined in DNAse I footprint assays. We demonstrated that a member of the NF-1/CTF family of transcription factors binds site II. Here we report the cloning of the protein that binds to enhancer site III. This protein is the rat homolog of human hCut/CDP, mouse Cux/CDP and canine Clox. hCut/Cux/CDP/Clox (hereafter called Cux/CDP), a 160 kDa protein containing multiple repeats and a homeodomain, negatively regulates the mammalian c-myc, gp91-phox and N-Cam genes. Using bacterially produced murine GST-Cux fusion proteins and GST-Cux deletion mutants, we find that Cux repeat CR3 and the homeodomain are both required for efficient binding to enhancer site III. Mouse lung and testis nuclear Cux/CDP bind to site III as determined in electrophoretic gel mobility supershift assays using two different anti-hCut specific monoclonal antibodies. Transfections of CAT constructs containing the enhancer fragment linked to a minimal promoter demonstrated that Cux/CDP represses c-mos enhancer activity.