Localization of the Blue-Light Receptor Phototropin to the Flagella of the Green Alga Chlamydomonas reinhardtii

Abstract

Blue light controls the sexual life cycle of Chlamydomonas, mediated by phototropin, a UV-A/blue-light receptor that plays a prominent role in multiple photoresponses. By using fractionation experiments and immunolocalization studies, this blue-light receptor, in addition to its known localization to the cell bodies, also was detected in flagella. Within the flagella, it was completely associated with the axonemes, in striking contrast to the situation in higher plants and the Chlamydomonas cell body where phototropin was observed in the plasma membrane. Its localization was not perturbed in mutants lacking several prominent structural components of the axoneme. This led to the conclusion that phototropin may be associated with the outer doublet microtubules. Analysis of a mutant (fla10) in which intraflagellar transport is compromised suggested that phototropin is a cargo for intraflagellar transport. The blue-light receptor thus seems to be an integral constituent of the flagella of this green alga, extending the list of organisms that harbor sensory molecules within this organelle to unicellular algae. INTRODUCTIONBlue light induces a variety of responses in higher and lower plants, many of which are mediated by photoreceptors of the phototropin family (Briggs and Huala, 1999;Christie and Briggs, 2001;Briggs and Christie, 2002). These photoreceptors mediate blue-light responses such as phototropism, chloroplast relocation, leaf expansion, rapid inhibition of hypocotyl growth, and stomatal opening.Phototropins are UV-A/blue-light receptors that are characterized by their common domain structure of two FMNbinding and light-sensing Per Arnt Sim domains, designated as LOV1 and LOV2, where LOV stands for light, oxygen, and voltage. These domains comprise the amino-terminal half of the protein; a serine/threonine protein kinase catalytic domain is localized to the carboxy-terminal half of the molecule Briggs and Christie, 2002).Excitation of the photoreceptor by blue light induces a cyclic photoreaction that involves the formation of a covalent adduct between the FMN chromophore and an activesite cysteine residue conserved in LOV1 and LOV2 (Salomon et al., 2000;Kottke et al., 2003). A resulting conformational alteration of the protein is believed to activate the protein kinase catalytic domain, resulting in autophosphorylation at multiple serine residues (Salomon et al., 2003). Correlative evidence suggesting an essential role for this activity in the signaling pathway has been collected (reviewed in Short and Briggs, 1994).A single phototropin gene (Phot) that encodes a protein with a structure typical for that of other members of the phototropin family was identified in Chlamydomonas reinhardtii (Huang et al., 2002). The functional role of the gene product was investigated using the RNA interference method that allowed the generation of strains with drastically reduced levels of phototropin. In these strains, progression of the sexual life cycle at light-requiring steps (the formation of mating-competent gametes ...

A phytochrome–phototropin light signaling complex at the plasma membrane

“…Light-directed growth (phototropism) in lower plants occurs in response to R and is mediated by phytochrome, phototropism in higher plants occurs in response to B and is mediated by phototropin photoreceptors. Although phototropins show no features characteristic of membrane proteins, they are plasma membrane-associated in higher plants and also Chlamydomonas (16,17,32,33). Here we show that all four Physcomitrella phototropins are associated with the plasma membrane ( Fig.…”

A NIMA-related kinase, Cnk2p, regulates both flagellar length and cell size inChlamydomonas

“…Another axonemal protein might also regulate commitment size: phototropin, a blue-light receptor in Chlamydomonas, has been localized to the axoneme and, like Cnk2p-HA, this localization is not disturbed in mutants lacking accessory axonemal structures (Huang et al, 2004). Blue light treatment increases the size threshold for commitment (Oldenhof et al, 2004).…”

Cyanidioschyzon merolae Genome. A Tool for Facilitating Comparable Studies on Organelle Biogenesis in Photosynthetic Eukaryotes

“…Recently, the function of two rhodopsins, Chlamydomonas sensory rhodopsins A and B, as phototaxis receptors was demonstrated by in vivo analysis of photoreceptor electrical currents and motility responses (Sineshchekov et al, 2002). In addition, blue light controls the sexual life cycle of Chlamydomonas, which is mediated by phototropin, a UV-A/blue-light receptor that plays a prominent role in multiple photoresponses (Huang and Beck, 2003;Huang et al, 2004). It is noteworthy that the mechanism of light signal transduction and the effect on cells differ between C. merolae and C. reinhardtii, as both are photosynthetic alga.…”