Brassinosteroid insensitive 1 (BRI1) is the major

receptor Receptor may refer to: * Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ...

of the

plant hormone Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pa ...

brassinosteroid. 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, promotes

pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

development, controls

vasculature The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...

development and promotes chilling and

freezing tolerance Freezing tolerance describes the ability of plants to withstand subzero temperatures through the formation of ice crystals in the xylem and intercellular space, or apoplast, of their cells. Freezing tolerance is enhanced as a gradual adaptation to l ...

. 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 This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

side of the membrane lies a series of 25 leucine-rich repeats (LRRs). The LRR

domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function * Do ...

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 A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequentl ...

section. The intracellular domain of BRI1 functions as a

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

and it is this domain triggers the

phosphorylation cascade A phosphorylation cascade is a sequence of signaling pathway events where one enzyme phosphorylates another, causing a chain reaction leading to the phosphorylation of thousands of proteins. This can be seen in signal transduction of hormone messag ...

that results in changes of

gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

Activation

In the absence of brassinosteroid, BRI1 is held in an inactive state by another protein, BRI1 kinase inhibitor 1 (BKI1). 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). 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. 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 factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...

Related proteins

BRI1-family proteins

In the model plant species

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...

, BRI1 acts alongside two

homologous Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...

proteins, known as BRI1-LIKE1 (BRL1) and BRL3. 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. Both BRL1 and BRL3 are able to bind brassinosteroids and act as receptors. A fourth BRI1-family protein, BRL2 cannot bind brassinosteroid and its function is unknown.

FLS2

BRI1 belongs to the large leucine-rich receptor-like protein kinase family. There are many other members of this family of proteins, and one of the most important is

FLS2 ''FLS'' genes have been discovered to be involved in flagellin reception of bacteria. FLS1 was the original gene discovered shown to correspond with a specific ecotype within ''Arabidopsis thaliana''. Even so, further studies have shown a second FL ...

. FLS2 acts as a detector of the bacterial protein

flagellin Flagellin is a globular protein that arranges itself in a hollow cylinder to form the filament in a bacterial flagellum. It has a mass of about 30,000 to 60,000 daltons. Flagellin is the principal component of bacterial flagella, and is present ...

and is important for

plant immunity Plant disease resistance protects plants from pathogens in two ways: by pre-formed structures and chemicals, and by infection-induced responses of the immune system. Relative to a susceptible plant, disease resistance is the reduction of pathoge ...

. Surprisingly (given their different functions) the signal cascades of BRI1 and FLS2 share many of the same components. 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.

References

{{Authority Control Plant hormones Plant development