Glutamic Acid Residues of Bacteriorhodopsin at the Extracellular Surface as Determinants for Conformation and Dynamics as Revealed by Site-Directed Solid-State ¹³ C NMR

We recorded 13 C NMR spectra of [3- 13 C]Ala- and [1- 13 C]Val-labeled bacteriorhodopsin (bR) and a variety of its mutants, E9Q, E74Q, E194Q/E204Q (2Glu), E9Q/E194Q/E204Q (3Glu), and E9Q/E74Q/E194Q/E204Q (4Glu), to clarify contributions of the extracellular (EC) Glu residues to the conformation and dynamics of bR. Replacement of Glu-9 or Glu-74 and Glu-194/204 at the EC surface by glutamine(s) induced significant conformational changes in the cytoplasmic (CP) surface structure. These changes occurred in the C-terminal α-helix and loops, and also those of the EC surface, as viewed from 13 C NMR spectra of [3- 13 C]Ala- and [1- 13 C]Val-labeled proteins. Additional conformational changes in the transmembrane α-helices were induced as modified retinal-protein interactions for multiple mutants involving the E194Q/E204Q pair. Significant dynamic changes were induced for the triple or quadruple mutants, as shown by broadened 13 C NMR peaks of [1- 13 C]Val-labeled proteins. These changes were due to acquired global fluctuation motions of the order of 10 -4 -10 -5 s as a result of disorganized trimeric form. In such mutants 13 C NMR signals from Val residues of [1- 13 C]Val-labeled triple and quadruple mutants near the CP and EC surfaces (including 8.7-Å depth from the surface) were substantially suppressed, as shown by comparative 13 C NMR studies with and without 40 μM Mn 2+ ion. We conclude that these Glu residues at the EC surface play an important role in maintaining the native secondary structure of bR in the purple membrane.