Brassinosteroid insensitive 1 (BRI1) is the major

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

of the

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

brassinosteroid Brassinosteroids (BRs or less commonly BS) are a class of polyhydroxysteroids that have been recognized as a sixth class of plant hormones and may have utility as anticancer drugs for treating endocrine-responsive cancers by inducing apoptosis of ...

. It plays very important roles in plant development, especially in the control of

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

elongation and for the tolerance of environmental stresses. BRI1 enhances cell elongation, promotes

pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...

development, controls

vasculature In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart an ...

development and promotes chilling and freezing tolerance. BRI1 is one of the most well studied hormone receptors and it acts a model for the study of

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...

-bound receptors in plants.

Structure

BRI1 is an integral

membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...

. 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

lies a series of 25

leucine Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α-Car ...

-rich repeats (LRRs). The LRR

domain A domain is a geographic area controlled by a single person or organization. Domain may also refer to: Law and human geography * Demesne, in English common law and other Medieval European contexts, lands directly managed by their holder rather ...

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 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 frequently u ...

section. The

intracellular 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 ...

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 mess ...

that results in changes of

gene expression Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...

Activation

In the absence of brassinosteroid, BRI1 is held in an inactive state by another protein,

BRI1 kinase inhibitor 1 Brassinosteroid insensitive 1 (BRI1) is the major Receptor (biochemistry), receptor of the plant hormone brassinosteroid. It plays very important roles in plant development, especially in the control of Plant cell, cell elongation and for the tole ...

(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 BRI1-associated receptor kinase 1 (BAK1- also known as somatic embryogenesis receptor kinase 3 or SERK3) is an important plant protein that has diverse functions in plant development. Structure BAK1 belongs to a large group of plant proteins k ...

(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 In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writt ...

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 Brassinosteroids (BRs or less commonly BS) are a class of polyhydroxysteroids that have been recognized as a sixth class of plant hormones and may have utility as anticancer drugs for treating endocrine-responsive cancers by inducing apoptosis of ...

(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 (genetics), transcription of genetics, genetic information from DNA to messenger RNA, by binding t ...

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 plant from the mustard family (Brassicaceae), native to Eurasia and Africa. Commonly found along the shoulders of roads and in disturbed land, it is generally ...

, BRI1 acts alongside two homologous 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 In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence ...

. FLS2 acts as a detector of the bacterial protein

flagellin Flagellins are a family of proteins present in flagellated bacteria which arrange themselves in a hollow cylinder to form the filament in a bacterial flagellum. Flagellin has a mass on average of about 40,000 daltons. Flagellins are the princi ...

and is important for

plant immunity Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars fro ...

. 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