Brassinosteroid insensitive-1

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Brassinosteroid insensitive-1 (BRI1) is the main receptor for brassinosteroids, the plant growth hormones. It plays a key role in making cells grow longer, in pollen development, vasculature (the plant’s veins) formation, and helping plants tolerate cold and other stresses.

BRI1 sits in the cell membrane. Its outside part has a series of 25 leucine-rich repeats that form a curved, horseshoe shape to bind brassinosteroids at a special site. Inside the cell, BRI1 has a kinase domain that starts a signaling cascade to change gene activity.

When brassinosteroid is absent, BRI1 is kept inactive by a partner protein called BKI1. When the hormone binds, BRI1 releases BKI1 and binds to another membrane protein called BAK1. Together, BRI1 and BAK1 act as a co-receptor and activate each other by phosphorylation.

Active BRI1 then phosphorylates downstream molecules called receptor-like cytoplasmic kinases (RLCKs), especially the BSK and CDG1 families. These signals are passed on to transcription factors that turn genes on or off.

In Arabidopsis, BRI1 works with two related proteins, BRL1 and BRL3, which mainly help vascular development. BRL1 and BRL3 can also bind brassinosteroids and act as receptors. A fourth family member, BRL2, can’t bind brassinosteroids, and its function is not clear. BRI1 is part of a large family of receptor-like kinases that includes FLS2, another important receptor that detects bacterial flagellin. Even though BRI1 and FLS2 have different roles, they share many signaling components, and it’s thought they may operate in different membrane microdomains to yield different responses.