The phototropins PHOT1 and 2 are blue light receptors that control blue light-induced processes such as phototropism, chloroplast relocation and stomatal opening (Briggs and Huala 1999; Jarillo et al. 1998). Both proteins have been shown to act redundantly, with PHOT1 being active under all light fluence rate conditions, and PHOT2 functioning specifically under high fluence rate conditions (Sakai et al. 2001; Matsuoka and Tokutomi 2005). PHOT1 and 2 consist of a light perception domain, comprising two light, oxygen or voltage (LOV) motifs that non-covalently bind the blue light sensitive molecule flavin mononucleotide (Huala et al. 1997), and a protein serine/threonine kinase domain (Reymond et al. 1992; Huala et al. 1997).
The molecular mechanism of phototropin action has been partly elucidated. Phototropins are membrane-associated proteins, and in the dark the kinase domain is repressed through binding of the photoreceptor domain. Light induces a conformational change in the photoreceptor that activates its protein kinase activity and at the same time releases the kinase from the plasma membrane (Sakamoto and Briggs 2002; Matsuoka and Tokutomi 2005). The BTB/POZ domain proteins nonphototropic hypocotyl 3 (NPH3) and root phototropism 2 (RPT2) were shown to be additional components in phototropism signaling (Motchoulski and Liscum 1999; Sakai et al. 2000; In-ada et al. 2004; Haga et al. 2005), and due to their domain structure they are believed to mediate protein-protein interactions. PHOT1, NPH3 and RPT2 are all associated with the plasma membrane and can bind to each other (Motchoulski and Liscum 1999; Inada et al. 2004; Sakamoto and Briggs 2002). A rice ortholog of NPH3 called coleoptile phototropism 1 was shown to act upstream of the redistribution of auxin induced by unilateral illumination of the seedling (Haga et al. 2005). Interestingly, NPH3 appears to be a phospho-protein that is in the phosphorylated state in the dark and becomes rapidly de-phosphorylated in the light. The preliminary finding that NPH3 interacts with CULLIN3 suggests that this protein is involved in targeting proteins for degradation by the proteasome (Pedmale and Liscum 2007). Although these observations place these proteins in early stages of this blue light-triggered signaling pathway, the molecular mechanism initiated by light and ultimately leading to PIN-mediated differential auxin transport and auxin signaling is still far from clear (Friml et al. 2002; Esmon et al. 2006).
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