Mutational analysis of Deinococcus radiodurans bacteriophytochrome reveals key amino acids necessary for the photochromicity and proton exchange cycle of phytochromes.
The ability of phytochromes (Phy) to act as photointerconvertible light switches in plants and microorganisms depends on key interactions between the bilin chromophore and the apoprotein that promote bilin attachment and photointerconversion between the spectrally distinct red light-absorbing Pr conformer and far red light-absorbing Pfr conformer. Using structurally guided site-directed mutagenesis combined with several spectroscopic methods, we examined the roles of conserved amino acids within the bilin-binding domain of Deinococcus radiodurans bacteriophytochrome with respect to chromophore ligation and Pr/Pfr photoconversion. Incorporation of biliverdin IXalpha (BV), its structure in the Pr state, and its ability to photoisomerize to the first photocycle intermediate are insensitive to most single mutations, implying that these properties are robust with respect to small structural/electrostatic alterations in the binding pocket. In contrast, photoconversion to Pfr is highly sensitive to the chromophore environment. Many of the variants form spectrally bleached Meta-type intermediates in red light that do not relax to Pfr. Particularly important are Asp-207 and His-260, which are invariant within the Phy superfamily and participate in a unique hydrogen bond matrix involving the A, B, and C pyrrole ring nitrogens of BV and their associated pyrrole water. Resonance Raman spectroscopy demonstrates that substitutions of these residues disrupt the Pr to Pfr protonation cycle of BV with the chromophore locked in a deprotonated Meta-R(c)-like photoconversion intermediate after red light irradiation. Collectively, the data show that a number of contacts contribute to the unique photochromicity of Phy-type photoreceptors. These include residues that fix the bilin in the pocket, coordinate the pyrrole water, and possibly promote the proton exchange cycle during photoconversion.
Wagner
Jeremiah R
JR
Departments of Genetics and Bacteriology, University of Wisconsin, Madison, Wisconsin 53706, USA.
Zhang
Junrui
J
von Stetten
David
D
Günther
Mina
M
Murgida
Daniel H
DH
Mroginski
Maria Andrea
MA
Walker
Joseph M
JM
Forest
Katrina T
KT
Hildebrandt
Peter
P
Vierstra
Richard D
RD
D019943:Amino Acid Substitution; D000596:Amino Acids / Q000378:metabolism*; D001664:Biliverdine / Q000378:metabolism; D001665:Binding Sites; D017124:Conserved Sequence; D004252:DNA Mutational Analysis; D034301:Deinococcus / Q000737:chemistry*; D005453:Fluorescence; D057927:Hydrophobic and Hydrophilic Interactions; D050505:Mutant Proteins / Q000737:chemistry; D009154:Mutation / Q000235:genetics; D010834:Phytochrome / Q000737:chemistry* / Q000235:genetics*; D011487:Protein Conformation; D011522:Protons; D011758:Pyrroles / Q000378:metabolism; D013059:Spectrum Analysis, Raman
D000596:Amino Acids; D050505:Mutant Proteins; D011522:Protons; D011758:Pyrroles; D010834:Phytochrome; D001664:Biliverdine