13C NMR study on conformation and dynamics of the transmembrane alpha-helices, loops, and C-terminus of [3-13C]Ala-labeled bacteriorhodopsin.

We have recorded 13C CP-MAS and DD-MAS NMR spectra of untreated and deionized [3-13C]-Ala-labeled bacteriorhodopsin (bR) and those cleaved with carboxypeptidase A and papain to gain insight into the conformation and dynamics of the transmembrane alpha-helices, loops, and C-terminus. It turned out that the C-terminus does not contribute to the 13C CP-MAS NMR spectra of [3-13C]Ala-bR recorded at ambient temperature owing to its rapid reorientational motions, since the relative peak intensities were unchanged in spite of the enzymatic cleavages. Therefore, the 13C CP-MAS NMR peaks of bR should be ascribed both to the transmembrane alpha-helices and loops. We further distinguished the peaks of the alpha II-helix form at 16.3 ppm (60%) from those of the alpha I-helix form at 14.9 ppm (20%) by deconvolution of the respective peaks of the hydrated [3-13C]Ala-bR, as referred to the 13C chemical shift of polyalanine in hexafluoroisopropyl alcohol. The remaining CP-MAS NMR peak of [3-13C]Ala-bR at 17.2 ppm was ascribed to the loops (20%) taking a variety of turn structures. In contrast, the 13C NMR signals from the C-terminal residues were significantly enhanced by recording the dipolar-decoupled (DD)-MAS NMR spectra. Conformational features of the two different portions of the C-terminus, residues 245-248 and 231-244, were revealed by the conformation-dependent 13C signals of bR successively cleaved by carboxypeptidase A and papain, respectively. The terminal end, residues 245-248, containing two Ala residues is virtually disordered and undergoing rapid motions.(ABSTRACT TRUNCATED AT 250 WORDS)