Edwards, Devin T.Hussain, SunyiaStone, Katherine M.Kinnebrew, MaiaKaminker, IliaMatalon, ErezSherwin, Mark S.Goldfarb, DaniellaHan, SongiHuber, Thomas2015-12-100969-2126http://hdl.handle.net/1885/63783The structural organization of the functionally relevant, hexameric oligomer of green-absorbing proteorhodopsin (G-PR) was obtained from double electron-electron resonance (DEER) spectroscopy utilizing conventional nitroxide spin labels and recently developed Gd3+-based spin labels. G-PR with nitroxide or Gd3+ labels was prepared using cysteine mutations at residues Trp58 and Thr177. By combining reliable measurements of multiple interprotein distances in the G-PR hexamer with computer modeling, we obtained a structural model that agrees with the recent crystal structure of the homologous blue-absorbing PR (B-PR) hexamer. These DEER results provide specific distance information in a membrane-mimetic environment and across loop regions that are unresolved in the crystal structure. In addition, the X-band DEER measurements using nitroxide spin labels suffered from multispin effects that, at times, compromised the detection of next-nearest neighbor distances. Performing measurements at high magnetic fields with Gd3+ spin labels increased the sensitivity considerably and alleviated the difficulties caused by multispin interactions.201410.1016/j.str.2014.09.0082015-12-10