Cytoplasmic surface structures of bacteriorhodopsin modified by site-directed mutations and cation binding as revealed by ¹³C NMR

Abstract.

We have examined how cytoplasmic surface structures of [3-¹³C]Ala-labeled bacteriorhodopsin (bR), consisting of the C-terminal α-helix and cytoplasmic loops, are altered by site-directed mutations at the former (R227Q) and the latter (A160G, E166G, and A168G) and by cation binding, by means of displacements of the ¹³C NMR peaks of Ala228 and Ala233 (C-terminal α-helix), Ala103 (C-D loop), and Ala160 (E-F loop). Cytoplasmic ends of the B and F helices were found to undergo fluctuation motions on the order of 10^–5 s, when such surface structures were disrupted, as viewed from suppressed ¹³C NMR signals. This happens also for deionized blue membranes of wild type and A160G, with accelerated fluctuations in the loops. Further, cytoplasmic surface structures of Na⁺-regenerated purple membrane from the blue membrane were significantly modified by Ca²⁺ ions up to 1 mM under relatively low ionic strength of 10 mM NaCl, although they are very similar at high ionic strength (100 mM NaCl). To interpret these findings, the following two surface structures were proposed. The C-terminal α-helix of the wild type at ambient temperature is involved in a perturbed type, probably tilted toward the direction of the B and F helices, to prevent unnecessary fluctuations of these helices for efficient proton uptake during the photocycle. An unperturbed type of helix is achieved when such a surface structure was disrupted at low temperature or in an M-like state. This view is consistent with previously published data for the "proton binding cluster" consisting of Asp104, Glu166, and Glu234.