Tube representation of the FXYD1 structure in the frame of the membrane (gray box), and of the alignment tensor (Sxx, Syy, Szz). (A) Helix H4 is nearly parallel to Szz. This is consistent with helix reorientation around Szz, resulting both in a GDO value similar to that of helices H1–H3, as well as greater dynamics observed from the heteronuclear NOEs and resonance intensities. (B) Helix H4 viewed from the C-terminus. A maximum helix reorientation of ρ≈ 40°, around Szz, maintains the amphiphilic polarity of hydrophobic and hydrophilic side-chains at the membrane surface.