The nature and kinetics of the conformational changes leading to the activated state of NpSRII/NpHtrII157 were investigated by time‐resolved electron paramagnetic resonance (TR‐EPR) spectroscopy in combination with site‐directed spin labeling (SDSL) on a series of spin labeled mutants of NpSRII. A structural rearrangement of the cytoplasmic moiety of NpSRII upon light activation was detected (helices B, C, F and G). The increase in distance between helices C and F in the M‐trapped state of the complex observed in one double mutant is in line with the notion that an outward movement of helix F occurs upon receptor activation. The data obtained from the NpSRII/NpHtrII157 complex reconstituted in purple membrane lipids are compared with those obtained from the X‐ray structure of the late M‐state of the complex which shows some discrepancies. The results are discussed in the context also of other biophysical and EPR experimental evidences.

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