BRI1-associated receptor kinase 1 (BAK1- also known as somatic embryogenesis receptor kinase 3 or SERK3) is an important plant

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

that has diverse functions in plant development.

Structure

BAK1 belongs to a large group of plant proteins known as the

Leucine-rich repeat A leucine-rich repeat (LRR) is a protein structural motif that forms an α/β horseshoe tertiary structure, fold. It is composed of repeating 20–30 amino acid stretches that are unusually rich in the hydrophobic amino acid leucine. These Pr ...

receptor kinases (LRR-RKs). 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 ...

'', BAK1 and 4 other closely related proteins form a sub-group within the LRR-RK family, known as the somatic embryogenesis receptor kinases (SERKs). All 5 SERKs are

transmembrane proteins 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 un ...

. They consist of an

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 ...

domain, a single transmembrane pass and an

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. The extracellular domain is composed of several leucine rich repeats, and the intracellular domain functions as a protein

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 ...

. BAK1 is thought to interact with many other LRR-RKs and the signalling output of BAK1 is dependent on its binding partner

Roles in plant development

Brassinosteroid signalling

BAK1 was initially identified for its role in

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 ...

signalling. Brassinosteroid is a major

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 ...

that has many roles and is often associated with cell elongation. BAK1 binds to the brassinosteroid receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) and this then triggers a phosphorylation cascade that leads to a change in expression of multiple genes.

Defense signalling

BAK1 is also critical for plant immunity and plants the lack BAK1 show a much greater susceptibility to bacterial infections. Plants are able to perceive

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

by recognizing specific molecular signatures or ' effectors'. One of these signatures is bacterial-derived

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 ...

. Plants perceive flagellin when it binds to the receptor

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 ...

. When flagellin is perceived by FLS2, this strongly promotes the interaction between FLS2 and BAK1 and this then leads to changes in gene expression that promote defense against bacteria. In addition to flagellin, plants are also able to perceive other bacterial effectors. One of these, EF-Tu is perceived by the EF-Tu receptor (EFR). Similar to FLS2, BAK1 is required for EFR function. The LRR-RK Erecta (ER) may also promote plant defense, in concert with BAK1.

Regulation of stomatal development

Along with its role in plant defense, the ER:BAK1 complex also repress the development of

stomata In botany, a stoma (: stomata, from Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange between the internal air spa ...

in leaves. Mutations in BAK1 and ER lead in increase in the number of stomata.

Molecular basis for diverse functional roles

The diverse functional roles of BAK1 are brought about through its binding to a large number of receptors. However, many of the molecular components downstream of these receptor:BAK1 complexes are shared between these signalling pathways (for example, BR Signalling Kinase 1 (BSK1) is a positive regulator of both BRI1:BAK1 signalling and FLS2:BAK1 signalling). It is currently unclear how cells are able to distinguish between a BSK1 which has been activated by BRI1 or FLS2. One recent study has shown that BRI1 and FLS2 localize to different 'nano-domains' on the cell membrane and so it is possible that this spatial separation accounts for the very different signal outputs.

References

External links

* {{MeSH name, BKI1+protein,+Arabidopsis Arabidopsis thaliana