Large Spectral Change due to Amide Modes of a β-Sheet upon the Formation of an Early Photointermediate of Middle Rhodopsin

Abstract

Rhodopsin contains retinal as the chromophore within seven transmembrane helices. Recently, we found a unique rhodopsin (middle rhodopsin, MR), which is evolutionarily located between the well-studied bacteriorhodopsin and sensory rhodopsin II, and which accommodates three retinal isomers in its ground state (the all-trans, the 13-cis, and, uniquely, the 11-cis isomers). In this study, we investigated structural changes of both the protein moiety and the retinal chromophore during photocycles of MR by time-resolved Fourier-transform infrared spectroscopy. Three photointermediates with decay time constants of 95 μs, 0.9 ms, and >~10 ms were identified by the global exponential fitting analysis. The first and third intermediates were attributed to the all-trans photocycle, in accordance with recently published results, whereas the second intermediate was likely one that was spectroscopically silent in the visible region and that was formed between the first and third states or resulted from the activation of the 13-cis isomer. By comparing light-induced difference spectra with various isotope labels in either the retinal or the protein moiety, we concluded that a β-sheet structure in the hydrophilic part was significantly altered during the all-trans photocycle of MR, which may involve an active state of the protein. This feature is characteristic of MR among microbial (type-1) rhodopsins.