{{Short description|Plant hormone receptor}} {{Infobox nonhuman protein | Name = Brassinosteroid insensitive 1 | image = | width = | caption = | Organism = Arabidopsis thaliana | TaxID = 3702 | Symbol = BRI1 | AltSymbols = | ATC_prefix = | ATC_suffix = | ATC_supplemental = | CAS_number = | CAS_supplemental = | DrugBank = | EntrezGene = 830095 | HomoloGene = 91895 | PDB = | RefSeqmRNA = NM_120100.3 | RefSeqProtein = NP_195650.1 | UniProt = O22476 | ECnumber = 2.7.11.1 | Chromosome = 4 | EntrezChromosome = NC_003075.7 | GenLoc_start = 18323800 | GenLoc_end = 18329486 }} thumb|400x400px|Representation of the BRI1-BAK1 co-receptor complex '''Brassinosteroid insensitive 1''' (BRI1) is the major receptor of the plant hormone brassinosteroid.<ref name="Wang_2001">{{cite journal | vauthors = Wang ZY, Seto H, Fujioka S, Yoshida S, Chory J | title = BRI1 is a critical component of a plasma-membrane receptor for plant steroids | journal = Nature | volume = 410 | issue = 6826 | pages = 380–3 | date = March 2001 | pmid = 11268216 | doi = 10.1038/35066597 | bibcode = 2001Natur.410..380W | s2cid = 4412000 }}</ref><ref name="Caño-Delgado_2004" /> It plays very important roles in plant development, especially in the control of cell elongation and for the tolerance of environmental stresses. BRI1 enhances cell elongation,<ref name="Wang_2001" /> promotes pollen development,<ref>{{cite journal | vauthors = Ye Q, Zhu W, Li L, Zhang S, Yin Y, Ma H, Wang X | title = Brassinosteroids control male fertility by regulating the expression of key genes involved in Arabidopsis anther and pollen development | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 13 | pages = 6100–5 | date = March 2010 | pmid = 20231470 | pmc = 2851861 | doi = 10.1073/pnas.0912333107 | bibcode = 2010PNAS..107.6100Y | doi-access = free }}</ref> controls vasculature development<ref name="Caño-Delgado_2004" /> and promotes chilling and freezing tolerance.<ref>{{cite journal | vauthors = Eremina M, Unterholzner SJ, Rathnayake AI, Castellanos M, Khan M, Kugler KG, May ST, Mayer KF, Rozhon W, Poppenberger B | title = Brassinosteroids participate in the control of basal and acquired freezing tolerance of plants | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 113 | issue = 40 | pages = E5982–E5991 | date = October 2016 | pmid = 27655893 | doi = 10.1073/pnas.1611477113 | pmc = 5056081 | bibcode = 2016PNAS..113E5982E | doi-access = free }}</ref> BRI1 is one of the most well studied hormone receptors and it acts a model for the study of membrane-bound receptors in plants.
== Structure == BRI1 is an integral membrane protein. On the extracellular side of the membrane lies a series of 25 leucine-rich repeats (LRRs). The LRR domain forms a horseshoe shape. An atypical LRR within this domain acts as the brassinosteroid binding site. Next to the LRR domain there is a single-pass transmembrane section. The intracellular domain of BRI1 functions as a kinase and it is this domain triggers the phosphorylation cascade that results in changes of gene expression.<ref name="Belkhadir_2015">{{cite journal | vauthors = Belkhadir Y, Jaillais Y | title = The molecular circuitry of brassinosteroid signaling | journal = The New Phytologist | volume = 206 | issue = 2 | pages = 522–40 | date = April 2015 | pmid = 25615890 | doi = 10.1111/nph.13269 | doi-access = free }}</ref>
== Activation == In the absence of brassinosteroid, BRI1 is held in an inactive state by another protein, BRI1 kinase inhibitor 1 (BKI1).<ref>{{cite journal | vauthors = Wang J, Jiang J, Wang J, Chen L, Fan SL, Wu JW, Wang X, Wang ZX | title = Structural insights into the negative regulation of BRI1 signaling by BRI1-interacting protein BKI1 | journal = Cell Research | volume = 24 | issue = 11 | pages = 1328–41 | date = November 2014 | pmid = 25331450 | pmc = 4220157 | doi = 10.1038/cr.2014.132 }}</ref> When brassinosteroid binds to BRI1, it reduces the stability of the BRI1:BKI1 complex and promotes the binding of BRI1 to another membrane protein, BRI1-associated receptor kinase 1 (BAK1).<ref name="Nam_2002">{{cite journal | vauthors = Nam KH, Li J | title = BRI1/BAK1, a receptor kinase pair mediating brassinosteroid signaling | journal = Cell | volume = 110 | issue = 2 | pages = 203–12 | date = July 2002 | pmid = 12150928 | doi=10.1016/s0092-8674(02)00814-0| doi-access = free }}</ref> In the BRI1:BAK1 complex, both BRI1 and BAK1 make contact with the brassinosteroid molecule and for this reason they are considered a co-receptor.<ref name="Belkhadir_2015" /><ref name="Nam_2002" /> BRI1 and BAK1 sequentially phosphorylate each other in their kinase domains, which results in the activation of BRI1. The activated kinase domain of BRI1 phosphorylates several receptor-like cytoplasmic kinases (RLCKs), notably the brassinosteroid signalling kinase (BSK) and constituitive differential growth 1 (CDG1) families. The RLCKs transduce this signal to downstream components, which ultimately results in the activation or de-activation of several transcription factors.<ref name="Belkhadir_2015" />
== Related proteins ==
=== BRI1-family proteins === In the model plant species Arabidopsis thaliana, BRI1 acts alongside two homologous proteins, known as BRI1-LIKE1 (BRL1) and BRL3.<ref name="Caño-Delgado_2004">{{cite journal | vauthors = Caño-Delgado A, Yin Y, Yu C, Vafeados D, Mora-García S, Cheng JC, Nam KH, Li J, Chory J | title = BRL1 and BRL3 are novel brassinosteroid receptors that function in vascular differentiation in Arabidopsis | journal = Development | volume = 131 | issue = 21 | pages = 5341–51 | date = November 2004 | pmid = 15486337 | doi = 10.1242/dev.01403 | doi-access = free | hdl = 11336/43673 | hdl-access = free }}</ref> The function of BRL1 and BRL3 appears to be restricted to the development of the vasculature system, but even in this context, BRI1 plays a more dominant role.<ref name="Caño-Delgado_2004" /> Both BRL1 and BRL3 are able to bind brassinosteroids and act as receptors.<ref name="Caño-Delgado_2004" /><ref name="Belkhadir_2015" /> A fourth BRI1-family protein, BRL2 cannot bind brassinosteroid and its function is unknown.<ref name="Belkhadir_2015" />
=== FLS2 === BRI1 belongs to the large leucine-rich receptor-like protein kinase family. There are many other members of this family of proteins,<ref>{{cite journal | vauthors = Smakowska-Luzan E, Mott GA, Parys K, Stegmann M, Howton TC, Layeghifard M, Neuhold J, Lehner A, Kong J, Grünwald K, Weinberger N, Satbhai SB, Mayer D, Busch W, Madalinski M, Stolt-Bergner P, Provart NJ, Mukhtar MS, Zipfel C, Desveaux D, Guttman DS, Belkhadir Y | display-authors = 6 | title = An extracellular network of Arabidopsis leucine-rich repeat receptor kinases | journal = Nature | volume = 553 | issue = 7688 | pages = 342–346 | date = January 2018 | pmid = 29320478 | doi = 10.1038/nature25184 | bibcode = 2018Natur.553..342S | pmc = 6485605 }}</ref> and one of the most important is FLS2.<ref name="Gómez-Gómez_2000">{{cite journal | vauthors = Gómez-Gómez L, Boller T | title = FLS2: an LRR receptor-like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis | journal = Molecular Cell | volume = 5 | issue = 6 | pages = 1003–11 | date = June 2000 | pmid = 10911994 | doi = 10.1016/S1097-2765(00)80265-8 | doi-access = free }}</ref> FLS2 acts as a detector of the bacterial protein flagellin and is important for plant immunity.<ref name="Gómez-Gómez_2000" /> Surprisingly (given their different functions) the signal cascades of BRI1 and FLS2 share many of the same components.<ref>{{cite journal | vauthors = Chinchilla D, Zipfel C, Robatzek S, Kemmerling B, Nürnberger T, Jones JD, Felix G, Boller T | title = A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence | journal = Nature | volume = 448 | issue = 7152 | pages = 497–500 | date = July 2007 | pmid = 17625569 | doi = 10.1038/nature05999 | hdl = 11858/00-001M-0000-0012-3840-F | bibcode = 2007Natur.448..497C | s2cid = 2818791 | hdl-access = free }}</ref> Recently it was suggested that BRI1 and FLS2 localize to different 'nano-domains' on the cell membrane and it is this spatial separation that accounts for their very different signal outputs.<ref>{{cite journal | vauthors = Bücherl CA, Jarsch IK, Schudoma C, Segonzac C, Mbengue M, Robatzek S, MacLean D, Ott T, Zipfel C | title = Plant immune and growth receptors share common signaling components but localise to distinct plasma membrane nanodomains | journal = eLife | volume = 6 | date = March 2017 | pmid = 28262094 | pmc = 5383397 | doi = 10.7554/eLife.25114 | doi-access = free }}</ref>
== References == {{Reflist}}
{{Authority Control}} Category:Plant hormones Category:Plant development