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Tongue Refolding in the Knotless Cyanobacterial Phytochrome All2699
journal contribution
posted on 2020-06-01, 13:06 authored by Qian-Zhao Xu, Lukas Goett-Zink, Wolfgang Gärtner, Kai-Hong Zhao, Tilman KottkePhytochromes regulate
central responses of plants and microorganisms
such as shade avoidance and photosystem synthesis. Canonical phytochromes
comprise a photosensory module of three domains. The C-terminal phytochrome-specific
(PHY) domain interacts via a tongue element with the bilin chromophore
in the central GAF (cGMP phosphodiesterase/adenylate cyclase/FhlA)
domain. The bilin isomerizes upon illumination with red light, transforming
the receptor from the Pr state to the Pfr state. The “knotless”
phytochrome All2699 from the cyanobacterium Nostoc sp. PCC7120 comprises three GAF domains as a sensory module and
a histidine kinase as an effector. GAF1 and GAF3 both bind a bilin,
and GAF2 contains a tongue-like element. We studied the response of
All2699, GAF1-GAF2, and GAF1 to red light by Fourier transform infrared
difference spectroscopy, including a 13C-labeled protein
moiety for assignment. In GAF1-GAF2, a refolding of the tongue from
β-sheet to α-helix and an upshift of the ring D carbonyl
stretch from 1700 to 1712 cm–1 were observed. Therefore,
GAF1-GAF2 is regarded as the smallest model system available to study
the tongue response and interaction with the chromophore. Replacement
of an arginine in the tongue with proline (R387P) did not affect the
unfolding of the β-sheet to Pfr but strongly impaired α-helix
formation. In contrast, the Y55H mutation close to bilin ring D did
not interfere with conversion to Pfr. Strikingly, the presence of
GAF3 in the full-length All2699 diminished the response of the tongue
and generated the signal pattern found for GAF1 alone. These results
point to a regulatory or integrative role of GAF3 in All2699 that
is absent in canonical phytochromes.