We have recorded 13C NMR spectra of the [3-13C]Ala, [1-13C]Val-labeled pharaonis transducer pHtrll(1-159) in the presence and absence of phoborhodopsin (ppR or sensory rhodopsin II) in egg phosphatidylcholine or dimyristoylphosphatidylcholine bilayers by means of site-directed (amino acid specific) solid-state NMR. Two kinds of 13C NMR signals of [3-13C]Ala-pHtrll complexed with ppR were clearly seen with dipolar decoupled magic angle spinning (DD-MAS) NMR. One of these resonances was at the peak position of the low-field α-helical peaks (αII-helix) and is identified with cytoplasmic α-helices protruding from the bilayers; the other was the high-field α-helical peak (αI-helix) and is identified with the transmembrane α-helices. The first peaks, however, were almost completely suppressed by cross-polarization magic angle spinning (CP-MAS) regardless of the presence or absence of ppR or by DD-MAS NMR in the absence of ppR. This is caused by an increased fluctuation frequency of the cytoplasmic α-helix from 105 Hz in the uncomplexed states to >106 Hz in the complexed states, leading to the appearance of peaks that were suppressed because of the interference of the fluctuation frequency with the frequency of proton decoupling (105 Hz), as viewed from the 13C NMR spectra of [3-13C]Ala-labeled pHtrll. Consistent with this view, the 13C DD-MAS NMR signals of the cytoplasmic α-helices of the complexed [3-13C]Ala-pHtrll in the dimyristoylphosphatidylcholine (DMPC) bilayer were partially suppressed at 0°C due to a decreased fluctuation frequency at the low temperature. In contrast, examination of the 13C CP-MAS spectra of [1-13C]Val-labeled complexed pHtrll showed that the 13C NMR signals of the transmembrane α-helix were substantially suppressed. These spectral changes are again interpreted in terms of the increased fluctuation frequency of the transmembrane α-helices from 103 Hz of the uncomplexed states to 10 4 Hz of the complexed states. These findings substantiate the view that the transducers alone are in an aggregated or clustered state but the ppR-pHtrll complex is not aggregated. We show that 13C NMR is a very useful tool for achieving a better understanding of membrane proteins which will serve to clarify the molecular mechanism of signal transduction in this system.
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