Future research on the mechanism of the signal transfer from the photoreceptor to the cytoplasmic two-component system will be guided by structural information. Primarily, the structure of the sensory rhodopsins as well as that of the receptor/transducer complexes will be of great importance. Previous work on the crystallisation of BR and HR provides the technique required to accomplish this goal. These structures will allow a first detailed insight into the receptor-transducer binding interface (see note on pg 39). The exploitation of structural information of intermediates might give direct evidence for the mechanism of signal transfer and will supplement the results from EPR, FTIR and other kinetic experiments. These investigations showed that the disruption of the salt bridge between the protonated Schiff base and its counter-ion leads to conformational changes involving helix F, which in turn triggers a rotation of TM2. It will be of fundamental interest to understand the mechanism of signal transfer from the membrane to the cytoplasmic tip of the transducer, which are about 20 nm apart. This knowledge will not only be important for the phototaxis and chemotaxis receptors, but will have general implications for transmembrane signalling such as in the case of, e.g., tyrosine kinases.
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