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Phototropins also regulate the movement of chloroplasts within the cell, notably chloroplast avoidance. It was thought that this avoidance serves a protective function to avoid damage from intense light[1], however an alternate study argues that the avoidance response is primarily to increase light penetration into deeper
mesophyll layers in high light conditions[2].
Enzyme Activity
Phototropins have two distinct light, oxygen, or voltage regulated domains (LOV1, LOV2) that each bind
flavin mononucleotide (FMN).[3] The flavin mononucleotide is noncovalently bound to a LOV domain in the dark, but becomes covalently linked upon exposure to suitable light.[3] The formation of the bond is reversible once light is no longer present.[3] The forward reaction with light is not dependent on temperature, though low temperatures give increased stability of the covalent linkage, leading to a slower reversal reaction.[3]
Light excitation will lead to a conformational change within the protein, which allows for
kinase activity.[4] There is also evidence to suggest that phototropins undergo autophosphorylation at various sites across the enzyme.[3] Phototropins trigger signaling responses within the cell, but it is unknown which proteins are phosphorylated by phototropins, or exactly how the autophosphorylation events play a role in signaling.[3]
Phototropins are typically found on the
plasma membrane, but some phototropins have been found in substantial quantities on chloroplast membranes.[5] One study found that phototropins on the plasma membrane play a role in phototropism, leaf flattening, stomatal opening, and chloroplast movements, while phototropins on the chloroplasts only partially affected stomatal opening and chloroplast movement,[6] suggesting that the location of the protein in the cell may also play a role in its signaling function.
^Kasahara, M., Kagawa, T., Olkawa, K., Suetsugu, N., Miyao, M., & Wada, M. (2002). Chloroplast avoidance movement reduces photodamage in plants. Nature, 420(6917).
https://doi.org/10.1038/nature01213
^Wilson, S., & Ruban, A. v. (2020). Rethinking the influence of chloroplast movements on non-photochemical quenching and photoprotection. Plant Physiology, 183(3).
https://doi.org/10.1104/pp.20.00549
^
abcdefgŁabuz, J., Sztatelman, O., & Hermanowicz, P. (2022). Molecular insights into the phototropin control of chloroplast movements. In Journal of Experimental Botany (Vol. 73, Issue 18).
https://doi.org/10.1093/jxb/erac271
^Koyama, T., Iwata, T., Yamamoto, A., Sato, Y., Matsuoka, D., Tokutomi, S., & Kandori, H. (2009). Different role of the Jα helix in the light-induced activation of the LOV2 domains in various phototropins. Biochemistry, 48(32).
https://doi.org/10.1021/bi9009192
^Kong, S. G., Suetsugu, N., Kikuchi, S., Nakai, M., Nagatani, A., & Wada, M. (2013). Both phototropin 1 and 2 localize on the chloroplast outer membrane with distinct localization activity. Plant and Cell Physiology, 54(1).
https://doi.org/10.1093/pcp/pcs151
^Ishishita, K., Higa, T., Tanaka, H., Inoue, S. I., Chung, A., Ushijima, T., Matsushita, T., Kinoshita, T., Nakai, M., Wada, M., Suetsugu, N., & Gotoh, E. (2020). Phototropin2 contributes to the chloroplast avoidance response at the chloroplast-plasma membrane InterfAce1[CC-by]. Plant Physiology, 183(5).
https://doi.org/10.1104/pp.20.00059