Functional Heme Binding to the Intrinsically Disordered C-Terminal Region of Bach1, a Transcriptional Repressor

Functional Heme Binding to the Intrinsically Disordered C-Terminal Region of Bach1, a Transcriptional Repressor

KEI SEGAWA,^1,2 MIKI WATANABE-MATSUI,^3,4 TOSHITAKA MATSUI,⁵ KAZUHIKO IGARASHI³ and KAZUTAKA MURAYAMA^1,6

¹Division of Biomedical Measurements and Diagnostics, Graduate School of Biomedical Engineering, Tohoku University, Sendai, Miyagi, Japan
²Pharmaceutical Discovery Research Laboratories, Teijin Pharma Limited, Tokyo, Japan
³Department of Biochemistry, Graduate School of Medicine, Tohoku University, Sendai, Miyagi, Japan
⁴Japan Society for the Promotion of Science (JSPS), Tokyo, Japan
⁵Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi, Japan
⁶Laboratory for Protein Functional and Structural Biology, RIKEN Center for Biosystems Dynamics Research, Yokohama, Kanagawa, Japan

Heme is one of the key factors involved in the oxidative stress response of cells. The transcriptional repressor Bach1 plays an important role in this response through its heme-binding activity. Heme inhibits the transcriptional-repressor activity of Bach1, and can occur in two binding modes: 5- and 6-coordinated binding. The Cys-Pro (CP) motif has been determined to be the heme-binding motif of Bach family proteins. The sequence of Bach1 includes six CP motifs, and four CP motifs are functional. With the aim of elucidating the molecular mechanism of heme-Bach1 regulation, we conducted biophysical analyses focusing on the C-terminal region of mouse Bach1 (residues 631-739) which is located after the bZip domain and includes one functional CP motif. UV-Vis spectroscopy indicated that the CP motif binds heme via 5-coordinated bond. A mutant, which included a cysteine to alanine substitution at the CP motif, did not show 5-coordination, suggesting that this binding mode is specific to the CP motif. Surface plasmon resonance revealed that the binding affinity and stoichiometry of heme with the Bach1 C-terminal region were K_D= 1.37 × 10⁻⁵ M and 2.3, respectively. The circular dichroism spectrum in the near-UV region exhibited peaks for heme binding to the CP motif. No significant spectral shifts were observed in the far-UV region when samples with and without heme were compared. Therefore, disordered-ordered transition such as “coupled folding and binding” is not involved in the Bach1-heme system. Consequently, the heme response of this C-terminal region is accomplished by disorder-disorder conformational alteration.

keywords —— circular dichroism spectroscopy; Cys-Pro motif; heme; intrinsically disordered protein; protein conformation

2019 Tohoku University Medical Press

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Tohoku J. Exp. Med., 2019, 247, 153-159

Correspondence: Kazutaka Murayama, Division of Biomedical Measurements and Diagnostics, Graduate School of Biomedical Engineering, Tohoku University, 2-1 Seiryo-machi, Aoba-ku, Sendai, Miyagi 980-8575, Japan.

e-mail: kazutaka.murayama.d4@tohoku.ac.jp