Key issues in the photochemistry and signalling-state formation of photosensor proteins

Four families of photosensors (i.e., rhodopsins, phytochromes, xanthopsins and cryptochromes) exist, which vary widely in the degree to which we understand the molecular basis of their activity. Some of their members are ideal model systems for studying the structure–function relation of proteins, and the role of dynamics therein. The photochemistry of photosensor activation is based upon the cis↔trans isomerization of the chromophore. This configurational transition leads to the formation of a signalling state of sufficient stability to communicate the presence of photons to a downstream signal-transduction partner. In the xanthopsins it has been demonstrated that the exact nature of this signalling state is strongly dependent on the mesoscopic context of the sensor protein. The cryptochromes appear to challenge the photoisomerization rule.