The SAF-box domain of chromatin protein DEK.

DEK is an abundant chromatin protein in metazoans reaching copy numbers of several millions/nucleus. Previous work has shown that human DEK, a protein of 375 amino acids, has two functional DNA-binding domains, of which one resides in a central part of the molecule and contains sequences corresponding to the scaffold attachment factor-box (SAF-box) domain as found in a growing number of nuclear proteins. Isolated SAF-box peptides (amino acids 137-187) bind weakly to DNA in solution, but when many SAF-box peptides are brought into close proximity on the surface of Sephadex beads, cooperative effects lead to a high affinity to DNA. Furthermore, a peptide (amino acids 87-187) that includes a sequence on the N-terminal side of the SAF-box binds efficiently to DNA. This peptide prefers four-way junction DNA over straight DNA and induces supercoils in relaxed circular DNA just like the full-length DEK. Interestingly, however, the 87-187 amino acid peptide introduces negative supercoils in contrast to the full-length DEK, which is known to introduce positive supercoils. We found that two adjacent regions (amino acids 68-87 and 187-250) are necessary for the formation of positive supercoils. Our data contribute to the ongoing characterization of the abundant and ubiquitous DEK chromatin protein.