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¹ Biomedical Engineering Research Organization, Tohoku University, 2-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan
² Department of Biochemistry, Graduate School of Medicine, Tohoku University, 2-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan
³ Protein Research Group, Genomic Sciences Center, RIKEN Yokohama Institute, 1-7-22 Suehiro-cho, Tsurumi, Yokohama 230-0045, Japan

The BTB/POZ domain is known as a protein–protein interaction motif that mediates homodimer and higher order self-associations. Proteins containing the BTB domain exist throughout eukaryotes; however, there is little information about the mechanism that determines the oligomeric state of the BTB domain. To address this question, we have determined the X-ray structure of the mouse Bach1 BTB domain. The present structure is similar to the previously determined BTB domain folds, including the human Bach1 BTB domain; however, distinct structural features are present, such as a novel homodimer interaction surface. The homodimer formation was found to involve a novel hydrogen bond network and interactions between hydrophobic surfaces of the kinked N-terminus (N-hook) and the partner's C-terminal residues. The deletion of the N-hook resulted in the conversion of the homodimer into a monomer in solution, indicating that the N-hook promotes the homodimerization of the mBach1 BTB domain. We have also found that the BTB domain of Bach2, a protein highly related to Bach1, is present as a monomer due to a short peptide insertion at the N-hook. These results represent the first example of the key modulatory element of BTB domain homodimerization.

^aPresent address: Nagasaki University School of Medicine, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan.

^bPresent address: Graduate School of Biomedical Engineering, Tohoku University, 2-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan.

^* Correspondence: kmura{at}bme.tohoku.ac.jp