Figure 8. Schematic illustration of light-induced conformational changes within the transmembrane region of the 2:2-complex of NpSRII with NpHtrll viewed from the cytoplasm [84], According to distance changes in the signalling M-state conformation helix F moves outwardly in the direction of the neighbouring TM2, which in turn is rotated clockwise as indicated by the red arrows. Black areas represent the original positions. The inset shows a close up of the dimer interface, suggesting the relative orientations of V78R1 and L82R1 in the dark (grey) and the light-activated states (white). The numbers at the arrows depict the distances (in nm) between corresponding residues in the dark (black) and light (red) states. It should be noted that a small piston-like movement of TM2 cannot be excluded.

1.4.2 Molecular mechanism of the signal transfer

Conformational alterations within protein interfaces play a key role in activating signal transduction chains mediated by membrane receptors such as receptor tyrosine kinases [207] or two-component systems in chemo- and phototaxis [1]. Especially, the signal transfer mediated by seven helix receptors such as rhodopsin is of great medical interest and the elucidation of the mechanism of archaeal photoreception might shed light on these questions.

Conformational changes of pharaonis sensory rhodopsin are already manifest in the early K state of the photocycle [127,152], Taking the EPR data into account, the structural alterations might occur in the vicinity of helix F. In BR an outward movement of the cytoplasmic end of this helix was originally proposed based on electron diffraction studies [208]. Recent crystallo-graphic analysis of BR [209,210] suggested that structural changes in the cytoplasmic parts of helix G occur during the transition of M and N. Supporting results were obtained by Steinhoff and co-workers [205,206] who observed transient mobility changes of residues in the interface of helices F and G. For NpSRII analogous experiments also indicate an outward tilting of helix F; however, it is correlated with the early steps of the photocycle and sustained in time over at least three orders of magnitude until the O-intermediate decays to the initial state. A similar movement of helix F (in the rhodopsin nomenclature VII) was described for rhodopsin [211]. Apparently, this region of the seven helix receptors seems to be generally involved in transmembrane signal transduction.

The question of the assignment of the signalling state to photocycle intermediates has been addressed by phototaxis experiments using retinal analogues. For SRI the attractant signalling is governed by the lifetime of the S373 intermediate rather than by the frequency of photocycling [190]. In similar experiments Yan et al. provided evidence that the signalling site is activated during formation of M, but is only reset by the decay of O [212], This is in line with EPR measurements using the NpSRII/NpHtrll complex, which indicated that the rearrangement of TM2 to its original conformation occurs during the last step of the photocycle [84],

Conformational changes were analysed in more detail by comparing the inter-residual distances in the original ground state and in the M-state within the complex between NpSRII and its truncated transducer [84]. On light excitation of the NpSRII/Htrll complex, significant distance changes are observed between the two TM2 and between TM2 and helix F of NpSRII, confirming the reported movement of the cytoplasmic part of helix F towards the transducer [108]. The pattern of the inter-helical distance changes allowed the establishment of a model of protein-protein signal transfer. The flap-like motion of helix F induces a clockwise rotation of TM2 (Figure 8). This rotary mechanism (a combination with small piston-like movement cannot be excluded) might be the trigger for the activation of the cytoplasmic two-component system. Supporting evidence comes from experiments on the aspartate receptor from S. typhimurium, which shows that upon binding of a substrate the periplasmic extension of one TM2 moves towards the cell interior by about 1.6 A and tilts with an angle of 5° [213]. Moreover, Ottemann et al. [214] provided evidence of a piston-like movement (<2.5 A) of TM2. An interesting observation was made on the time course of the rotation in comparison to the helix F movement [84]. Clearly, the two structural changes were out of phase in the second half of the photocycle: TM2 reaches its original location only after helix F (as well as the chromophore) has returned back to the resting state. This decoupling permits the system to modulate the activation/deactivation of the transducer by changing the state of methylation or by allosteric regulation within a receptor/transducer network [215].

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