Biology:White Collar-1

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Short description: Blue light photoreceptor in fungi
White Collar-1
Identifiers
OrganismN. crassa
Symbolwc-1
Entrez3875924
RefSeq (mRNA)XM_011396849
RefSeq (Prot)XP_011395151
UniProtQ01371

White Collar-1 (wc-1) is a gene in Neurospora crassa encoding the protein WC-1 (127 kDa).[1][2] WC-1 has two separate roles in the cell. First, it is the primary photoreceptor for Neurospora[3] and the founding member of the class of principle blue light photoreceptors in all of the fungi.[4] Second, it is necessary for regulating circadian rhythms in FRQ. It is a key component of a circadian molecular pathway that regulates many behavioral activities, including conidiation.[5][6] WC-1 and WC-2, an interacting partner of WC-1, comprise the White Collar Complex (WCC) that is involved in the Neurospora circadian clock. WCC is a complex of nuclear transcription factor proteins, and contains transcriptional activation domains, PAS domains, and zinc finger DNA-binding domains (GATA).[7] WC-1 and WC-2 heterodimerize through their PAS domains to form the White Collar Complex (WCC).[8][9]

Discovery

The Neurospora circadian clock was discovered in 1959, when Pittendrigh et al. first described timing patterns in the asexual development of spores.[10] They noticed that in the region of the growing front, mycelia laid down between the late night to early morning formed aerial hyphae, whereas those laid down at other times did not.[10][11] This aerial growth pattern at subjective circadian times served as tentative support for the presence of circadian oscillators.

White Collar-1 was described in the 1960s by geneticists who saw a strain of Neurospora which the mycelia were albino, but the conidia were normally pigmented.[12]

The mutant gene was designated white collar (wc), for the white coloration of the mycelia below the pigmented conidia on agar slants.[13] The gene wc-1 was classified during the mapping of the chromosomal loci of Neurospora crassa (1982 by Perkins et al.).[14]

Protein structure

WC-1 and WC-2 are transcription factors encoded by the genes wc-1 and wc-2. Zinc finger DNA-binding domains (GATA) allow WC-1 and WC-2 to bind to DNA and act as transcription factors.[1][8]

Both WC-1 and WC-2 have PAS domains that allow them to bind to other proteins.[10][15] WC-1 and WC-2 typically heterodimerize in vivo to form the White Collar Complex (WCC), which acts as a transcription factor complex. In vitro, WC-1 can also homodimerize with itself and heterodimerize with other PAS proteins.[10]

Protein sequencing has revealed WC-1 to also contain a LOV domain, a chromophore-binding peptide region.[2][16] This binding site is highly conserved, and is sequentially similar to the chromophore-binding domains of phototropins in plants.[17]

WC-1 and WC-2 bind to the promoter elements of the genes that they transcriptionally activate.[3][10]

Function in the circadian clock

Photoreception

WC-1 has been shown to be a blue-light photoreceptor, and is a necessary component of the Neurospora light-induced response pathway.[3] Genetic screens of light-insensitive Neurospora mutant strains have repeatedly demonstrated abnormalities in the wc-1 gene.[16] In functional Neurospora, the WC-1 LOV domain binds to the flavin adenine dinucleotide (FAD) chromophore,[3][18] which is responsible for the conversion of light to mechanical energy. FAD displays maximum absorption of light at 450 nm,[19] thus explaining WC-1's maximal sensitivity to blue light.

Light-induced responses are completely eliminated in WC-1 LOV knockout Neurospora mutants, although WC-1's transcription activation role persists in the dark.[16] WC-1 is widely conserved among fungi where it appears to be the principle blue light photoreceptor for the entire kingdom.[4]

Circadian regulation

Simplified Representation of Neurospora Circadian Clock.[6] WC-1 and WC-2 (WCC) act as positive activators of frq transcription. FRQ protein binds to an RNA helicase, FRH, and to CK1 forming a complex.[20] This complex interacts with WCC, promoting phosphorylation of WCC. Phosphorylation stabilizes WCC and promotes its export to the cytoplasm, effectively down-regulating frq transcription.[21]

The White Collar Complex (WCC), the heterodimer of WC-1 and WC-2, acts as a positive element in the circadian clock. WCC serves as an activator of frq gene transcription by binding to two DNA promotor elements in the nucleus: the Clock box (C box) and the Proximal Light-Response Element (PLRE). PLRE is required for maximal light induction, while the C box is required for both maximal light induction and maintaining circadian rhythmicity in constant darkness.[10][22]

In addition, light-activated WCC is shown to induce the transcription of VIVID, a small flavin-binding blue-light photoreceptor that is required for adaptation to light-induced responses in the transcription of light-induced genes, including wc-1 and frq.[23][24][25] VIVID is a negative regulator of light responses, although its mechanism is not yet known.[26]

As part of the transcription-translation negative feedback-loop (TTFL), FRQ protein enters the nucleus and interacts with FRQ-interacting RNA Helicase (FRH) to promote the repression of WCC activity. This FRQ–FRH complex is suggested to recruit protein kinases such as casein kinase (CK I) and CK II to phosphorylate WCC.[27] The phosphorylation of the WCC stabilizes WCC, preventing it from binding and activating frq transcription. Protein phosphatases PP2A and PP4 are known to counterbalance kinase activity and support the reactivation and nuclear entry of WCC.[28]

FRQ has also been shown to interact with WC-2 in vitro, and a partial loss-of-function allele of wc-2 yields Neurospora with a long period length and altered temperature compensation, which is a key characteristic of circadian pacemakers.[10][29]

Only WC-1 is required for transient light-induction, but both WC-1 and WC-2 are required for the circadian clock to run.[10][30] Because the core of the clock is based on a rhythmic expression of frq, the acute light-induction pathway provides a direct way to reset the clock. Mammalian clocks can be reset through a similar mechanism, via the light-induction of the mammalian per genes within the SCN.[31]

Several WC-1 mutants are known. The rhy-2 mutation was localized to the polyglutamine region of the WC-1 gene product. The rhy-2 mutant is arrhythmic with regard to conidiation in constant darkness, but rhythmic in a light-dark cycle. Rhy-2 is also only weakly sensitive to light, suggesting that the polyglutamine region may be essential for both clock function and light sensing in Neurospora.[32]

See also

References

  1. ↑ 1.0 1.1 "White collar-1, a central regulator of blue light responses in Neurospora, is a zinc finger protein". The EMBO Journal 15 (7): 1650–7. Apr 1996. doi:10.1002/j.1460-2075.1996.tb00510.x. PMID 8612589. 
  2. ↑ 2.0 2.1 "Interconnected feedback loops in the Neurospora circadian system". Science 289 (5476): 107–110. Jul 2000. doi:10.1126/science.289.5476.107. PMID 10884222. Bibcode: 2000Sci...289..107L. 
  3. ↑ 3.0 3.1 3.2 3.3 "White Collar-1, a circadian blue light photoreceptor, binding to the frequency promoter". Science 297 (5582): 815–9. Aug 2002. doi:10.1126/science.1073681. PMID 12098706. Bibcode: 2002Sci...297..815F. 
  4. ↑ 4.0 4.1 "How fungi keep time: circadian system in Neurospora and other fungi". Current Opinion in Microbiology 9 (6): 579–87. Dec 2006. doi:10.1016/j.mib.2006.10.008. PMID 17064954. 
  5. ↑ "The circadian clock of Neurospora crassa". FEMS Microbiology Reviews 36 (1): 95–110. Jan 2012. doi:10.1111/j.1574-6976.2011.00288.x. PMID 21707668. 
  6. ↑ 6.0 6.1 "Neurospora wc-1 and wc-2: transcription, photoresponses, and the origins of circadian rhythmicity". Science 276 (5313): 763–9. May 1997. doi:10.1126/science.276.5313.763. PMID 9115195. 
  7. ↑ Fungal Genomics. Elsevier. 2003. 
  8. ↑ 8.0 8.1 "Roles for WHITE COLLAR-1 in circadian and general photoperception in Neurospora crassa". Genetics 163 (1): 103–14. Jan 2003. doi:10.1093/genetics/163.1.103. PMID 12586700. 
  9. ↑ "Role of a white collar-1-white collar-2 complex in blue-light signal transduction". The EMBO Journal 18 (18): 4961–8. Sep 1999. doi:10.1093/emboj/18.18.4961. PMID 10487748. 
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 "Molecular bases for circadian clocks". Cell 96 (2): 271–90. Jan 1999. doi:10.1016/S0092-8674(00)80566-8. PMID 9988221. 
  11. ↑ Pittendrigh, C. S.; Bruce, V. G.; Rosensweig, N. S.; Rubin, M. L. (Jul 1959). "Growth Patterns in Neurospora: A Biological Clock in Neurospora". Nature 184 (4681): 169–170. doi:10.1038/184169a0. Bibcode: 1959Natur.184..169P. 
  12. ↑ "New markers and map sequences in Neurospora crassa, with a description of mapping by duplication coverage, and of multiple translocation stocks for testing linkage". Genetica 40 (3): 247–78. 1969. doi:10.1007/BF01787357. PMID 5367562. 
  13. ↑ Harding, R W; Shropshire, W (1980). "Photocontrol of Carotenoid Biosynthesis". Annual Review of Plant Physiology 31 (1): 217–238. doi:10.1146/annurev.pp.31.060180.001245. 
  14. ↑ "Chromosomal loci of Neurospora crassa". Microbiological Reviews 46 (4): 426–570. Dec 1982. doi:10.1128/mr.46.4.426-570.1982. PMID 6219280. 
  15. ↑ "Role of a white collar-1-white collar-2 complex in blue-light signal transduction". The EMBO Journal 18 (18): 4961–4968. Sep 1999. doi:10.1093/emboj/18.18.4961. PMID 10487748. 
  16. ↑ 16.0 16.1 16.2 "Photoreception in Neurospora: a tale of two White Collar proteins". Cellular and Molecular Life Sciences 60 (10): 2131–2138. Oct 2003. doi:10.1007/s00018-003-3109-5. PMID 14618260. 
  17. ↑ "White collar-1, a DNA binding transcription factor and a light sensor". Science 297 (5582): 840–3. Aug 2002. doi:10.1126/science.1072795. PMID 12098705. Bibcode: 2002Sci...297..840H. 
  18. ↑ "[Photoreceptor apparatus of a fungus Neurospora crassa]" (in ru). Molekuliarnaia Biologiia 39 (4): 602–617. Jul–Aug 2005. PMID 16083009. 
  19. ↑ "The FAD-dependent tricarballylate dehydrogenase (TcuA) enzyme of Salmonella enterica converts tricarballylate into cis-aconitate". Journal of Bacteriology 188 (15): 5479–5486. Aug 2006. doi:10.1128/JB.00514-06. PMID 16855237. 
  20. ↑ "Quantitative proteomics reveals a dynamic interactome and phase-specific phosphorylation in the Neurospora circadian clock". Molecular Cell 34 (3): 354–63. May 2009. doi:10.1016/j.molcel.2009.04.023. PMID 19450533. 
  21. ↑ "Dissecting the mechanisms of the clock in Neurospora". Circadian Rhythms and Biological Clocks, Part A. Methods in Enzymology. 551. 2015. pp. 29–52. doi:10.1016/bs.mie.2014.10.009. ISBN 9780128012185. 
  22. ↑ "Bright to dim oscillatory response of the Neurospora circadian oscillator". Journal of Biological Rhythms 29 (1): 49–59. Feb 2014. doi:10.1177/0748730413517983. PMID 24492882. 
  23. ↑ "VIVID is a flavoprotein and serves as a fungal blue light photoreceptor for photoadaptation". The EMBO Journal 22 (18): 4846–4855. Sep 2003. doi:10.1093/emboj/cdg451. PMID 12970196. 
  24. ↑ "The PAS protein VIVID defines a clock-associated feedback loop that represses light input, modulates gating, and regulates clock resetting". Cell 104 (3): 453–64. Feb 2001. doi:10.1016/s0092-8674(01)00232-x. PMID 11239402. 
  25. ↑ "Blue light adaptation and desensitization of light signal transduction in Neurospora crassa". Molecular Microbiology 39 (4): 1080–7. Feb 2001. doi:10.1046/j.1365-2958.2001.02306.x. PMID 11251826. 
  26. ↑ "Interlocked feedback loops of the circadian clock of Neurospora crassa". Molecular Microbiology 68 (2): 255–262. Apr 2008. doi:10.1111/j.1365-2958.2008.06148.x. PMID 18312266. 
  27. ↑ "FRQ-interacting RNA helicase mediates negative and positive feedback in the Neurospora circadian clock". Genetics 184 (2): 351–361. Feb 2010. doi:10.1534/genetics.109.111393. PMID 19948888. 
  28. ↑ "Methods to study molecular mechanisms of the Neurospora circadian clock". Circadian Rhythms and Biological Clocks, Part A. Methods in Enzymology. 551. 2015. pp. 137–151. doi:10.1016/bs.mie.2014.10.002. ISBN 9780128012185. 
  29. ↑ "Neurospora WC-1 recruits SWI/SNF to remodel frequency and initiate a circadian cycle". PLOS Genetics 10 (9): e1004599. Sep 2014. doi:10.1371/journal.pgen.1004599. PMID 25254987. 
  30. ↑ "Light-induced resetting of a circadian clock is mediated by a rapid increase in frequency transcript". Cell 81 (7): 1003–1012. Jun 1995. doi:10.1016/s0092-8674(05)80005-4. PMID 7600569. 
  31. ↑ "Light-induced resetting of a mammalian circadian clock is associated with rapid induction of the mPer1 transcript". Cell 91 (7): 1043–1053. Dec 1997. doi:10.1016/s0092-8674(00)80494-8. PMID 9428526. 
  32. ↑ "A new wc-1 mutant of Neurospora crassa shows unique light sensitivity in the circadian conidiation rhythm". Molecular Genetics and Genomics 268 (1): 56–61. Sep 2002. doi:10.1007/s00438-002-0722-1. PMID 12242499. 



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