W0062

Crystal Structures and Molecular Mechanism of a Light-Induced Signaling Switch: The Phot-LOV1 Domain. Ilme Schlichting^1,3, Roman Fedorov¹, Elisabeth Hartmann¹, Tatjana Domratcheva¹, Markus Fuhrmann², Peter Hegemann², ¹Max Planck Institut für Molekulare Physiologie, Abt. Biophysikalische Chemie, Otto Hahn Str. 11, 44227 Dortmund, Germany, ²Institut für Biochemie I, Univ. Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany, ³Max Planck Institut für Medizinische Forschung, Abt. Biomolekulare Mechanismen, Jahnstr. 29, 69120 Heidelberg, Germany.

Phot-proteins (phototropin and homologues) are blue light photoreceptors that control mechanical processes like phototropism, chloroplast relocation or guard cell opening in plants. Phot-receptors consist of two FMN-binding LOV (Light Oxygen Voltage) domains and a C-terminal serine/threonine kinase domain that autophosphorylates upon absorption of a blue light photon. The approaches used and difficulties encountered in determining the structures of the resting dark state and the unstable photoproduct of the LOV1-domain of Phot1 from the green alga Chlamydomonas reinhardtii will be described. Light absorption causes formation of a proposed active signaling state that is characterized by a covalent bond between the flavin C4a and the thiol of Cys57. In the resting dark state of LOV1 Cys57 is present in two conformations. The structures are compared with the one of LOV2 from the fern Phy3 (Crosson & Moffat), interpreted in the light of the spectroscopic data and used as a basis for quantum chemical calculations to obtain insight in the reaction mechanism. It will be presented and compared to previously suggested mechanisms.