Ta from Bak 86C and Bak 69C/111C in apoptotic mitochondria (Fig. 2) were consistent with the BGH structure determined here (Fig. 1). The EPR spectra of spin-labeled Avasimibe web residues attached to various locations of the BGH were very similar whether they were present in the tetrameric GFP-Bak in solution or in oligomeric Bak in membrane (Supplementary Information Figure S4f). Also, the distance get LDN193189 between 84R1s within a BGH domain remained essentially the same in the above two states (Supplementary Information Figure S3c). All these strongly suggest that the BGH structure in the oligomeric Bak pore in the membrane is very similar to the X-ray crystal structure of BGH observed in solution state, consistent with our previous report27. In the GFP-Bak tetramer, the two BGH units form a partly open hydrophobic pocket in which the hydrophobic surfaces are sequestered away from the surface and thus not readily available for interaction with the membrane (Fig.1d). Furthermore, between the two BGHs, the C-terminal residues of the two closer 3 helices are separated at a large distance ( 40 ? unlike what was observed in the membrane (Fig. 2). Thus, the `3/5 interface’ was implicated neither in the GFP-Bak tetramer nor in the crystal contacts (Supplementary Information Figure S1b). The immersion depths of the R1s in oligomeric Bak indicated that the BGH and 6 helices are adsorbed to the membrane surface at shallow depths (Fig. 4), consistent with others30. In our BGH structure, the two central 5 helices in the BGH form an angle of approximately 15 (?) degrees relative to a hypothetical horizontal plane that is set parallel to the 2- 3 helices (Fig. 4e). Assuming that BGH is immersed flat in the membrane, the helical tilt of 5 would be approximately 15 (?) degrees relative to the membrane surface. The membrane-immersion depths of 130R1, 138R1, 141R1 and 144R1 in 5 helix appear to be consistent with this assumption (Fig. 4d,e). Note that the immersion depth of a R1 side chain depends not only on the positionScientific RepoRts | 6:30763 | DOI: 10.1038/srepDiscussionwww.nature.com/scientificreports/Figure 4. Interaction of BH3-in-groove homodimer and 6 helix with membrane. (a) Membrane immersion depths of the nitroxide spin label side chains (R1s) in mouse Bak BGH and 6 helix domains in oligomeric Bak are shown as a function of residue locations (average values of 2? experiments with error ranges indicated). The sinusoidal curves represent the depth-fitting curves for residues 149?58 with (solid) or without (dotted) residue 157 (see Supplementary Information Figure S6c for details). The residues marked with dotted vertical lines correspond to the local maxima in depth. (b) The immersion depths of R1s in the hydrophobic surface of BGH in top (top) and side (bottom) views. Black spheres represent C-atoms of R1s. (c) Immersion depths and topological locations 6 residues in Bak in a helical wheel diagram. The direction of the greatest depth (see Supplementary Information Figure S6c) corresponds to the rotational orientation of the helix facing the membrane. The residues with a square mark correspond to those in tertiary contacts or in protein interior. The circled residues represent amino acid locations at which the accessibility parameter to oxygen, (O2), reaches a local maximum in each helical turn (see Supplementary Information Figure S6a). (d) Helix tilting angle and the topological locations of the indicated R1s in 5-6 region in oligomeric Bak are shown. Approx.Ta from Bak 86C and Bak 69C/111C in apoptotic mitochondria (Fig. 2) were consistent with the BGH structure determined here (Fig. 1). The EPR spectra of spin-labeled residues attached to various locations of the BGH were very similar whether they were present in the tetrameric GFP-Bak in solution or in oligomeric Bak in membrane (Supplementary Information Figure S4f). Also, the distance between 84R1s within a BGH domain remained essentially the same in the above two states (Supplementary Information Figure S3c). All these strongly suggest that the BGH structure in the oligomeric Bak pore in the membrane is very similar to the X-ray crystal structure of BGH observed in solution state, consistent with our previous report27. In the GFP-Bak tetramer, the two BGH units form a partly open hydrophobic pocket in which the hydrophobic surfaces are sequestered away from the surface and thus not readily available for interaction with the membrane (Fig.1d). Furthermore, between the two BGHs, the C-terminal residues of the two closer 3 helices are separated at a large distance ( 40 ? unlike what was observed in the membrane (Fig. 2). Thus, the `3/5 interface’ was implicated neither in the GFP-Bak tetramer nor in the crystal contacts (Supplementary Information Figure S1b). The immersion depths of the R1s in oligomeric Bak indicated that the BGH and 6 helices are adsorbed to the membrane surface at shallow depths (Fig. 4), consistent with others30. In our BGH structure, the two central 5 helices in the BGH form an angle of approximately 15 (?) degrees relative to a hypothetical horizontal plane that is set parallel to the 2- 3 helices (Fig. 4e). Assuming that BGH is immersed flat in the membrane, the helical tilt of 5 would be approximately 15 (?) degrees relative to the membrane surface. The membrane-immersion depths of 130R1, 138R1, 141R1 and 144R1 in 5 helix appear to be consistent with this assumption (Fig. 4d,e). Note that the immersion depth of a R1 side chain depends not only on the positionScientific RepoRts | 6:30763 | DOI: 10.1038/srepDiscussionwww.nature.com/scientificreports/Figure 4. Interaction of BH3-in-groove homodimer and 6 helix with membrane. (a) Membrane immersion depths of the nitroxide spin label side chains (R1s) in mouse Bak BGH and 6 helix domains in oligomeric Bak are shown as a function of residue locations (average values of 2? experiments with error ranges indicated). The sinusoidal curves represent the depth-fitting curves for residues 149?58 with (solid) or without (dotted) residue 157 (see Supplementary Information Figure S6c for details). The residues marked with dotted vertical lines correspond to the local maxima in depth. (b) The immersion depths of R1s in the hydrophobic surface of BGH in top (top) and side (bottom) views. Black spheres represent C-atoms of R1s. (c) Immersion depths and topological locations 6 residues in Bak in a helical wheel diagram. The direction of the greatest depth (see Supplementary Information Figure S6c) corresponds to the rotational orientation of the helix facing the membrane. The residues with a square mark correspond to those in tertiary contacts or in protein interior. The circled residues represent amino acid locations at which the accessibility parameter to oxygen, (O2), reaches a local maximum in each helical turn (see Supplementary Information Figure S6a). (d) Helix tilting angle and the topological locations of the indicated R1s in 5-6 region in oligomeric Bak are shown. Approx.