Cryptochrome By Vidan Biology

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1. CRYPTOCHROME

2. 2 TYPES OF PHOTORECEPTORS 1. Phytochrome (Red Light Photoreceptor) Plays a role in seed germination, flowering and stem elongation etc. 2. Cryptochrome (Blue Light Photoreceptor) Plays a role in flowering, circadian rhythms and inhibition of stem elongation etc. 3. Phototropin (Blue Light Photoreceptor) Plays a role in chloroplast movement and phototropism etc 4. Zeitlupe family (Blue Light Photoreceptor) plays a role in circadian rhythms and control of flowering. There are 4 major types of photoreceptors involved in photomorphogenesis :

3. 1 2 3 Cryptochrome Phototropin Zeaxanthin Photoreceptor Associated With Blue Light Response.

4. CRY1 It is located equally in both nucleus and cytoplasm. It is blue light photoreceptor of plant and promotes photomorphogenesis. CRY 2 CRY 2-It is exclusively localized in nucleus. It is blue light photoreceptor of plant and promotes photomorphogenesis . CRY 3 CRY 3-It is CRY-DASH type of CRY present in Mitochondria and chloroplast and functions to repair UV induced damage on ssDNA . It is not a blue light photoreceptor that promotes photomorphogenesis CRYPTOCHROME UNIQUE PROPERTIES Cryptochrome is structurally similar to photolyase a blue light activated Enzyme that repairs pyrimidine dimers in DNA. But cryptochrome do not have any role in DNA repairs, but they are thought to have evolved from photolyase enzyme ➤ In Arabidopsis, 3 cryptochrome genes are reported The major difference between CRY1 & CRY2 is that Cry1 is stable in the presence of blue light while CRY 2 undergoes degradation under blue light.

5. Cryptochrome is a dimeric flavoprotein dimerization is essential for their activity and it is mediated by the blue light sensory PHR domain. Cryptochrome undergoes blue light dependent homodimerization and photoactivation . In CRY blue light is absorbed by Pterin (MTHF) and is transferred to FAD. FAD is the primary chromophore regulating cryptochrome activity. Blue light is perceived by PHR domain & is transferred to CCT domain to start the signaling pathway.

6. CRYPTOCHROME SPECIAL PROPERTY Cryptochrome are structurally related to photolyase. Photolyase is a blue light activated enzyme that repairs pyrimidine dimer in DNA . The Cryptochrome do not have a DNA repair activity, but they are thought to have evolved from the photolyase . Cryptochrome play a very important role in the generation and maintenance of circadian rhythm(biological clock)

7. Photolyase homology region CCT DAS FAD MTHF N C Domain Structure Of Plant Cytochrome Cryptochrome consists of 2 domains : Amino Terminal Photolyase Homology region (PHR) which binds Chromophores non-covalently . It is sensor domain . A. Flavin which is FAD B. Pterin which is MTHF Cryptochrome Carboxyl Terminal (CCT) Domain - within the CCT there is a sub domain known as DAS at carboxyl terminal. It is the output domain .

8.   Dimerization is essential for biological activity and is mediated by N-terminal domain . The C-terminal domain transduce the light signal by interacting with the E3 ubiquitin ligase, COP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1) STRUCTURAL DOMAIN

9. MODELS OF CRYPTOCHROME There are two models of cryptochrome   Closed Model: Under dark conditions PHR domain binds CCT domain tightly repressing CCT domain activity in the absence of light 2. Open Model: Upon the absorption of blue light by PHR domain there is slight conformational change which results in homodimerization & autophosphorylation of CCT domain on multiple serine residues which causes electrostatic repulsion of CCT domain from PHR domain. That results in deattachment of CCT domain from PHR domain there by making multiple sites accessible for phosphorylation by unidentified kinase "Y".   So phosphorylation plays important role in derepression of CCT domain from PHR domain which results in its photoactivation of cryptochrome .

10. MUTATION CASES OF CRY1, CRY2 & COP1 CRY1 plays a primary role in de-etiolation of plants and CRY2 plays a primary role in photoperiodic flowering . Mutation of CRY1, CRY2 & COP11. Mutation in CRY2 led to delayed flowering under blue light. Overexpression of CRY2 leads to early flowering Mutation in CRY1 results in elongated hypocotyl and etiolated growth in blue light. Overexpression of CRY1 results in strong inhibition of hypocotyl and overproduction of anthocyanin. COP1 mutant displace constitutive photo morphogeneic phenotype in which dark grown seedlings mimic light grown seedlings.

11.   CRY1 plays a primary role in inhibition of stem elongation . CRY1 plays a primary role in leaf expansion .   CRY2 plays a primary role in promotion of photoperiodic flowering. CRY2 promotes photoperiodic flowering in Arabidopsis. CRY1 plays major role & to some extent CRY2 plays a role in setting circadian rhythm in plants and animals .   6. CRY1 promotes cotyledons expansion . 7. CRY1 promotes anthocyanin production .   8. CRY1 inhibits petiole elongation. 9. CRY1 also serve as magnetoreceptors in Animals. FUNCTIONS

12. MAIN FUNCTIONS 1. Help in Anthocyanin production . 2. Stomatal opening . 3. Circadian clock . 4. Cryptochrome are evolutionarily old and highly conserved proteins . 5. Phototropism 6. Photomorphogenesis 7. Light capture 8. Inhibition of stem elongation . 9. Circadian rhythm 10. Magnetoreception-sensing of magnetic fields in a number of special

13. CCT SPA1 COP1 LAF1 HY5 HFR1 Photomorphogenesis Kinase Y P Light Responsive gene + + Active Inactive CRY1 Active In mutant CRY1, N-terminal PHR domain is non-functional and CCT conformation even in the absence of light due to constitutive domain can form active phosphorylation. Active CCT domain inhibits COP1 SPA1 activity even in darkness there by preventing degradation of photomorphogenesis promoting transcription factors like HFR1 , HYS, LAF1 . So accumulation of HY5, HFR1 & LAF1 proteins in the nucleus promotes photomorphogenesis even in darkness. CCT overexpression mutant; Dark & Light Nucleus

14. LAF1 HY5 HFR1 Photomorphogenesis Kinase Y Light Responsive gene + + Active Nucleus PHR PHR Inactive Inactive Active CRY 1 Dimer Photoactivated CRY1 binds to SPA1 in COP1 SPA1 complex via CCT domain resulting in dissociation of COP1-SPA1 complex which inhibits COP1 activity . This prevents the degradation of photomorphogenesis promoting transcription factors like HY5, HFR1 & LAF1 . In Blue light CRY1 undergoes homo dimerization via PHR domain which partially activates CRY1 & promotes blue light dependent Autophosporylation of CCT domain & phosphorylation by unknown kinase "Y" which fully activates CRY1 in the nucleus . This promotes photomorphogenesis in presence of blue light. LIGHT

15. CCT PHR Inactive CRY 1 monomer In dark, CRY1 remains inactive in the nucleus in form of monomer . In dark, COP1 (E3 ubiquitin ligase) & SPA1 forms complex in the nucleus which positively regulates COP1 activity . COP1-SPA1 complex degrade photomorphogenesis promoting transcription factors like HY5,HFR1 & LAF1 via. Ubiquitin mediated 265 proteasome degradation . This inhibits photomorphogenesis and promotes etiolated growth of plants in dark. DARK Nucleus