Phototropin

Phototropins are blue light photoreceptor proteins (more specifically, flavoproteins) that mediate phototropism responses across many species of algae,[1] fungi and higher plants.

When phototropins are hit with blue light, they induce a signal transduction pathway that alters the plant cells' functions in different ways.

[4] In addition phototropins mediate the first changes in stem elongation in blue light prior to cryptochrome activation.

[11] Phototropins have two distinct light, oxygen, or voltage regulated domains (LOV1, LOV2) that each bind flavin mononucleotide (FMN).

[13] The FMN is noncovalently bound to a LOV domain in the dark, but becomes covalently linked upon exposure to suitable light.

The crystal structure of the LOV2 domain of Phototropin-2 of Arabidopsis thaliana , generated using ChimeraX. Part of the LOV2 domain is hidden for clarity of the active site containing FMN. The dotted blue lines represent hydrogen bonds predicted as important in binding. In green are Cys426 and Arg427 residues which are crucial in photoactivity and FMN binding, respectively, with mutations resulting in total loss of function of the protein. [ 12 ] Upon photoexcitation, the sulfur (yellow) of Cys426 forms a covalent bond with the carbon 4 of FMN. ( PDBe: 4EEP )