Phototropins are photoreceptor proteins (specifically, flavoproteins) that mediate phototropism responses in higher plants. Along with cryptochromes and phytochromes they allow plants to respond and alter their growth in response to the light environment. Phototropins may also be important for the opening of stomata.
Phototropins are autophosphorylating protein kinases that activate in response to blue light. When blue light hits the phototropin protein in the cell membrane, the phototropin protein will unfold and undergo phosphorylation that can cause a cascade of events inside of the cell.
Phototropins are part of the phototropic sensory system in plants that causes various environmental responses in plants. Phototropins specifically will cause stems to bend towards light, and stomata to open. Also, phototropins are important in chloroplast movements inside the cell. They also mediate the first changes in stem elongation in blue light (before cryptochromes become active) and phototropin 1 also is required for blue light mediated transcript destabilization of specific mRNAs in the cell.