No abstract

Local dynamics of interhelical loops in bacteriorhodopsin (bR), the extracellular BC, DE and FG, and cytoplasmic AB and CD loops, and helix B were determined on the basis of a variety of relaxation parameters for the resolved 13C and 15N signals of [1-13C]Tyr-, [15N]Pro- and [1-13C]Val-, [15N]Pro-labeled bR. Rotational echo double resonance (REDOR) filter experiments were used to assign [1-13C]Val-, [15N]Pro signals to the specific residues in bR. The previous assignments of [1-13C]Val-labeled peaks, 172.9 or 171.1 ppm, to Val69 were revised: the assignment of peak, 172.1 ppm, to Val69 was made in view of the additional information of conformation-dependent 15N chemical shifts of Pro bonded to Val in the presence of 13C-15N correlation, although no assignment of peak is feasible for 13C nuclei not bonded to Pro. 13C or 15N spin-lattice relaxation times (T1), spin-spin relaxation times under the condition of CP-MAS (T2), and cross relaxation times (TCH and TNH) for 13C and 15N nuclei and carbon or nitrogen-resolved, 1H spin-lattice relaxation times in the rotating flame (1H T1 rho) for the assigned signals were measured in [1-13C]Val-, [15N]Pro-bR. It turned out that V69-P70 in the BC loop in the extracellular side has a rigid beta-sheet in spite of longer loop and possesses large amplitude motions as revealed from 13C and 15N conformation-dependent chemical shifts and T1, T2, 1H T1 rho and cross relaxation times. In addition, breakage of the beta-sheet structure in the BC loop was seen in bacterio-opsin (bO) in the absence of retinal.

Structural changes in bacteriorhodopsin (bR) in two different processes of retinal reconstitutions were investigated by observing the (13)C and (15)N solid-state NMR spectra of [1-(13)C]Val- and [(15)N]Pro-labeled bR. We found that NMR signals of the BC loop were sensitive to changes in protein structure and dynamics, from wild-type (WT) bR to bacterio-opsin (bO), regenerated bR and E1001 bR. Regenerated bR was prepared following the addition of retinal into bO obtained from photobleached WT-bR. E1001 bR was cultured from a retinal-deficient strain termed E1001 following the addition of retinal to growing cells. (15)N NMR signal at Pro70 in the BC loop in WT-bR was observed at 122.4 p.p.m., whereas signals were not apparent or partly suppressed in bO and regenerated bR, respectively. Similarly, the (13)C NMR signal at Val69 in the BC loop at 172.0 p.p.m. that was observed in WT-bR was significantly decreased in both regenerated bR and bO. These results suggest that the dynamic structure of the BC loop in bO was substantially altered following the removal of retinal. As a consequence, the correct protein structure failed to be recovered via the regenerating process of retinal to bO. On the other hand, (13)C and (15)N NMR signals at the BC loop in E1001 bR appeared at positions identical to those of WT-bR. The results of the current study indicate that the BC loop may not always fold correctly in the regenerated bR, which leads to different properties in the regenerated bR compared to that of WT-bR.