AI Article Synopsis
- Phytochromes, such as Agp2 from Agrobacterium tumefaciens, are photoreceptors that switch between two forms (Pr and Pfr) based on light exposure, with Agp2 having a Pfr ground state due to a dark conversion.
- Agp2 exhibits unusual spectral properties in its Pr form, indicating a deprotonated chromophore, and the pKa changes significantly between the two forms (over 3 units), making its photoconversion sensitive to pH levels.
- Mutations and swap experiments in the domain structure of Agp2 affect the protonation state and the rate of conversion from Pr to Pfr, highlighting the role of specific protein domains in regulating phytochrome function
Article Abstract
Phytochromes are widely distributed photoreceptors with a bilin chromophore that undergo a typical reversible photoconversion between the two spectrally different forms, Pr and Pfr. The phytochrome Agp2 from Agrobacterium tumefaciens belongs to the group of bathy phytochromes that have a Pfr ground state as a result of the Pr to Pfr dark conversion. Agp2 has untypical spectral properties in the Pr form reminiscent of a deprotonated chromophore as confirmed by resonance Raman spectroscopy. UV/visible absorption spectroscopy showed that the pKa is >11 in the Pfr form and ∼7.6 in the Pr form. Unlike other phytochromes, photoconversion thus results in a pKa shift of more than 3 units. The Pr/Pfr ratio after saturating irradiation with monochromatic light is strongly pH-dependent. This is partially due to a back-reaction of the deprotonated Pr chromophore at pH 9 after photoexcitation as found by flash photolysis. The chromophore protonation and dark conversion were affected by domain swapping and site-directed mutagenesis. A replacement of the PAS or GAF domain by the respective domain of the prototypical phytochrome Agp1 resulted in a protonated Pr chromophore; the GAF domain replacement afforded an inversion of the dark conversion. A reversion was also obtained with the triple mutant N12S/Q190L/H248Q, whereas each single point mutant is characterized by decelerated Pr to Pfr dark conversion.
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| http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3814768 | PMC |
| http://dx.doi.org/10.1074/jbc.M113.479535 | DOI Listing |
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