Propagation of photoinduced electric field changes through phytochrome and their impact on conformational transitions

In phytochromes, photoisomerization of the chromophore and subsequent structural relaxations lead to the functionally essential secondary structure transition of the tongue, a phytochrome-specific protein segment. The coupling mechanism between chromophore and protein structural changes is yet not understood, but electric field changes are discussed to play an important role. In this work, electric field changes in the chromophore binding pocket (CBP) are confirmed to propagate over long distances through the protein and alter the electric field in the tongue region. An experimental-theoretical approach to analyze local electric fields using Stark reporters has been further developed. These are nitrile groups introduced site-specifically into the protein via noncanonical amino acids. The functional integrity of the variants is checked by crystallography and various spectroscopies. For the first time, functionally intact variants with substitutions in the tongue are generated. Based on frequency shifts and relative intensities of the nitrile stretching modes, hydrogen-bonding and noncovalent electric field contributions are separated. The field changes originating in the CBP are transduced to the tongue along a pathway via Phe192. Given a proper direction of the net electric field vector in the tongue region, the magnitude of the field may be sufficient to destabilize the tongue structure.