A LOV domain and its photocycle.

(A) The second light-oxygen-voltage (LOV) domain derived from Avena sativa phototropin 1 (AsLOV2) binds a flavin cofactor (FMN) to sense blue light. In the dark state, the C-terminal Jα helix of AsLOV2 is tightly bound to its core domain (switch-off, left panel). Upon irradiation with blue light, the Jα helix is released from the core domain of AsLOV2 (switch-on, right panel). When the blue light is turned off, the open conformation of AsLOV2 in the light state is returned back to its closed conformation in the dark state (right to left). The blue light-dependent conformational change of AsLOV2 switches the activity of an effector domain, such as a protein with enzymatic activity and a peptide, connected at the C-terminus of AsLOV2. (B) A photochemical reaction, known as a photocycle, occurring between a LOV domain and a flavin cofactor. Blue light irradiation induces the formation of a covalent bond between the thiol group of a cysteine within a LOV domain and the C4a position of the isoalloxazine ring of flavin (left to right). The photoadduct spontaneously breaks when the LOV domain is returned back to the dark condition (right to left). The photoadduct formation and its break lead to loss of fluorescence from the flavin cofactor and its recovery, respectively.