Surface and dynamic structures of bacteriorhodopsin in a 2D crystal, a distorted or disrupted lattice, as revealed by site-directed solid-state 13C NMR
The 3D structure of bacteriorhodopsin (bR) obtained by x-ray diffraction or cryo-electron microscope studies is not always sufficient for a picture at ambient temperature where dynamic behavior is exhibited. For this reason, a site-directed solid-state 13C NMR study of fully hydrated bR from purple membrane (PM), or a distorted or disrupted lattice, is very valuable in order to gain insight into the dynamic picture. This includes the surface structure, at the physiologically important ambient temperature. Almost all of the 13C NMR signals are available from [3-13C]Ala or [1-13C]Val-labeled bR from PM, although the 13C NMR signals from the surface areas, including loops and transmembrane α-helices near the surface (8.7Å depth), are suppressed for preparations labeled with [1-13C]Gly, Ala, Leu, Phe, Tyr, etc. due to a failure of the attempted peak-narrowing by making use of the interfered frequency of the frequency of fluctuation motions with the frequency of magic angle spinning. In particular, the C-terminal residues, 226-235, are present as the C-terminal α-helix which is held together with the nearby loops to form a surface complex, although the remaining C-terminal residues undergo isotropic motion even in a 2D crystalline lattice (purple membrane) under physiological conditions. Surprisingly, the 13C NMR signals could be further suppressed even from [3-13C]Ala- or [1-13C]Val-bR, due to the acquired fluctuation motions with correlation times in the order of 10-4 to 10-5 s, when the 2D lattice structure is instantaneously distorted or completely disrupted, either in photo- intermediate, removed retinal or when embedded in the lipid bilayers.
Photochemistry and Photobiology
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American Society of Photobiology
Author Posting. (c) The Authors (2007) This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in PHOTOCHEMISTRY AND PHOTOBIOLOGY, 83(2): 253-262. http://dx.doi.org/10.1562/2006.06-12-IR-917