LIGHT-DEPENDENT ON PHYTOCHROME PHOTOBODY PATTERN
“Phys in higher plants are red (R) and far-red (FR)
light receptors that use a linear tetrapyrrole phyto- chromobilin as their chromophore”. Phys can interconvert between two quite constant conformers, as R light is absorbed by stationary Pr form (lmax = 660) and a FR light is absorbed by active Pfr form (lmax =730) while it was firstly think that phys is restricted
mainly in the cytoplasm because series of studies were make over ten years ago using GUS-tagged and luminous protein-tagged phys persuasively verified that photoactivation from the Pr to the Pfr form result in the swift translocation of phys from the cytoplasm to the nucleus (Sakamoto and Nagatani). This change is localized and is one of the initial phy response to light for both phyA and phyB the most important phys in Arabidopsis in nuclear accumulation is necessary for most of their downstream response.
It was first report by Akira Nagatani that photoactivated phyB-GFP was not only restricted in the nucleus but also more divided in to categories to subnuclear speckle-like photobodies. Eberhard
Schafer, Ferenc Nagy and colleagues verified that all five Arabidopsis phys, phyA to E confine to photobodies in the light and that phy photobody localization is sealed in both dicotyledonous and monocotyledonous plants. While most studies on phy photobodies were accomplished by using transgenic lines that overexpressed luminous protein-tagged phys both Pisum- sativum phyA and Arabidopsis phyB photobodies have been observed using immunocytochemistery signifying of that the formation of photobodies is not an artifact of phy overexpression.
The translocation of phys to photobodies happens very rapidly during the dark to light alteration in photobodies that contain both phyA and phyB can be observed after 1 to 2 min of R light exposure. PhyB photobody localization is prompted by R light.
In distinction of phyA photobody localization is prompted by R, FR and blue light. These early photobodies are transitory and disappear after1 h of light exposure. Phy photobodies re-emerge after 2 h in R light and remain present in the light. These late photobodies contain mostly phyB because phyA is quickly despoiled in R light.
Joanne Chory and colleagues showed that the size and number of phyB photobodies under incessant R light is firmed by the percentage of phyB in the Pfr form at a given instant. Light conditions that shift the Pr/ Pfr balance to the Pfr side the Pfr form will prop up large phy photobody formation. Consistent with this perception under high intensity R light which force the equilibrium to the Pfr form phyB emerge to be contained completely to a few large photobodies with diameters between 1 and 2 mm. By contrast under dim R light with a low R to FR ratio where more phyB live in the Pr form, phyB tends to confine too many smaller photobodies or localizes diffusely in the nucleoplasm. The formation of large phyB photobodies link tightly with the light dependent hypocotyl reticence response. The fact that the steady shape model of phyB-GFP is conventional and can be accurately functioning by external light quantity and quality makes it an excellent visible read aloud for genetic screens. Even though phyB photobodies appear to be morphologically secure they are quite lively subnuclear domains fluorescence revitalization after photobleaching experiment on “phyB yellow fluorescent protein” photobodies showed that photobodyassociated phyB-YFP is quickly replaced with nucleoplasmic phyB-YFP.
Some phy photobodies may also have the blue light receptor cry2. Crys are photolyase like photoreceptors that use “FAD” as the chromophore. When phyB-GFP and cry2-red fluorescent protein were co-expressed in BY-2 protoplast not only did they co-localize on photobodies they could also be coimmunoprecipitated, because cry2 is photolabile and quickly tarnished in blue light. It could be co-localized with phyB in early photobodies during the dark to light conversion. Consistent with this hypothesis “cry2” photobody localization is also a quick light response as Arabidopsis “cry2” is translocated to photobodies within 15 min after blue light exposure.
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Activated “cry1” has also been recommended to focus on nuclear bodies because “cry1” and “cry2” were colocalized with“COP1”and “SPA1” on nuclear bodies. It is possible that “cry1” similar to “cry2” might also have to phy-containing photobodies in the light.
STRUCTURE OF PHOTOBODY LOCALIZATION AND PHYTOCHROME
The intramolecular necessities for photobody localization have been most widely studied using Arabidopsis “phyB”.
Two common approaches have been taken to analyze the phy subdomains essential for photobody localization. One approach is to observe the localization sample of phy truncation remains the other approach is to distinguish the localization trial of loss of functions of phy.
The domain formations of phys have been well defined. Phys can form also homodimers or heterodimers. Each monomer is an approximately125kD polypeptide.
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The crystal structure of the “PAS” and “GAF” area of the bacteriophytochrome “DrBph1” since Deinococcus radiodurans exposed the existence of a light sense knot which plays an essential role in signaling. For example amino acid residue situated within the knot is occupied in the relations with phytochrome interacting factors.
Truncation studies have exposed that the C-terminal half of phyB localizes to photobodies separately of light. In the same way the photobody localization of phyA also requires its C terminal half. Inside the C terminal domain of phyB and PRD is both essential and adequate for nuclear localization telling that it also possesses a nuclear localization signal and it is able to bind an unknown protein containing a NLS. The role of the “HKRD” in photobody localization is still unknown while truncations deficient the whole “HKRD” do not localize to photobodies a phyB truncation lack a segment of the “HKRD” might still form smaller photobodies. Because nuclear and photobody localization are both light dependent. The current model is that in the Pr form C terminal localization signals are covered by the N terminal domain in the course of an interaction between the “GAF” PHY subdomains and the “PRD” while both the accepted NLS and photobody localization signals are exposed in the Pfr form as a result of light dependent conformational changes. The open conformation of the Pfr form could also representation domains required for interacting with other signaling mechanism. Consistent with the impression that the conformation of Pfr is important for photobody localization the “NTE” which plays a role in stabilize the Pfr form is also required for phyB photobody formation. Localization studies of missense phyB alleles have more established that the Pfr form of phyB is required for photobody localization. N terminal missense loss of function phyB alleles that are imperfect in photobody localization also have less stable Pfr and some have irregular light absorption spectra. By contrast “YHB” a constitutively active phyB changed localizes to photobodies despite of light conditions. Moreover loss-of-function mutations that only affect signaling but not the absorption properties of phyB have normal photobody localization patterns which more suggests that the photobody localization depends on phyB being in the Pfr form and is not a significance of
Phy signaling.
ROLE OF PHOTOBODIES IN
LIGHT SIGNALING
Ever as the initial observation of phy photobodies there has been much thought about their function. One hypothesis is that the photobodies are storage depots for active photoreceptors but are not required for light signaling. In this model photobodies provide as a regulator to regulate the amount of active phy in the nucleoplasm.
Most phy-mediated response requires global reprogramming of the transcriptome. Two emerging signaling mechanisms suggest that the key signaling events regulating gene expression work by modulating the permanence of either positively or negatively acting transcription factors. The positively acting transcription factors include the vital leucine zipper transcription factor.
The current model is that phys support the stability of this group of positively acting transcription regulators by repressing E3. Although the molecular mechanism ligases Chen and Chory 2011 how phys repress “COP1’ complex is still unclear recently it has been shown that crys directly regulate either the formation of the substrate receptor “COP1” and “SPA1” complex or the interaction between the substrate receptor “COP1” and “SPA” complex and its target proteins. It is quite possible that phys could develop a similar mechanism to adjust the activity of “COP1”. Further positively acting transcriptional regulator there are also transcriptional regulators that are opposed to phy signaling. Some of the well studied member of this group is the “bHLH” transcription factor called “PIFs”. Phys bind directly to PIFs and activate their phosphorylation and consequent deprivation in the light. The rapid turnover of “PIFs” in the light is a key mechanism to turn on phy-mediated responses. One generally proposed assumption is that photobodies are sites for protein degradation. This model is support by the reality that many signaling mechanism are localized to photobodies former to their degradation. For example during the dark to light transition in seedling improvement in phyA and PIF3 colocalize to early phy photobodies before their degradation. In addition the positively performing transcriptional regulators including “HY5”, “LAF1”, “HFR1”, and some “BBX” proteins also colocalize with “COP1” on nuclear bodies.
