CCAAT displacement activity involves CUT repeats 1 and 2, not the CUT homeodomain.

The CCAAT displacement protein, the homolog of the Drosophila melanogaster CUT protein, contains four DNA-binding domains: three CUT repeats (CR1, CR2, and CR3) and the CUT homeodomain (HD). Using a panel of fusion proteins, we found that a CUT repeat cannot bind to DNA as a monomer, but that certain combinations of domains exhibit high DNA-binding affinity: CR1+2, CR3HD, CR1HD, and CR2HD. One combination (CR1+2) exhibited strikingly different DNA-binding kinetics and specificities. CR1+2 displayed rapid on and off rates and bound preferably to two C(A/G)AT sites, organized as direct or inverted repeats. Accordingly, only CR1+2 was able to bind to the CCAAT sequence, and its affinity was increased by the presence of a C(A/G)AT site at close proximity. A purified CCAAT displacement protein/CUT protein exhibited DNA-binding properties similar to those of CR1+2; and in nuclear extracts, the CCAAT displacement activity also required the simultaneous presence of a C(A/G)AT site. Moreover, CR1+2, but not CR3HD, was able to displace nuclear factor Y. Thus, the CCAAT displacement activity requires the presence of an additional sequence (CAAT or CGAT) and involves CR1 and CR2, but not the CUT homeodomain.