Molecule of the Month: Phytohormone Receptor DWARF14

Many hormones deliver their signals by binding to receptors on cell surfaces, launching a cascade of signaling enzymes inside the cell. Familiar examples include insulin and glucagon, the hormones that help regulate blood sugar. Steroid hormones like estrogen and testosterone travel inside a cell and bind to receptor proteins, which then bind to DNA and regulate gene expression. Some of the phytohormones used by plants have an entirely different mode of action. These phytohormones bind to an enzyme in the cell, which then activates the ubiquitin/proteasome system, regulating key proteins involved in growth by destroying them.

Strigolactones are small molecules synthesized in plant cells from carotenoids. They bind to a receptor called DWARF14 (D14 for short), so named because mutations in the gene produce smaller plants with more branching. D14 is an enzyme that breaks strigolactones into pieces. In the process, one piece remains attached to the enzyme, blocking its enzymatic action but activating it as a receptor (see the Jmol interactive below). The activated D14 then binds to the MAX2 protein, which then binds to a subunit of the ubiquitination machinery, SKP1. Ultimately, this will create an active ubiquitination complex that will proceed to degrade the relevant regulatory proteins.

PDB ID 5hzg captures the process after strigolactone has been bound and cleaved, and the MAX2 and SKP1 proteins have been bound. The result is a large complex poised to bring together the ubiquitination machinery and proteins that are its targets. D14 may also be a one-shot enzyme. Once it cleaves a strigolactone molecule and builds this complex, it may no longer be effective as an enzyme and will itself be degraded after it’s finished delivering its messages.

Compounds created during forest fires act as plant growth regulators in a similar way. Karrikins are formed by the burning of cellulose and sugars, and fall to the ground in smoke particles. Later, they are washed into the soil by rain, they enter buried seeds, and bind to a receptor called KAI2. This receptor helps control the germination of seeds, so binding of karrikins can help spur sprouting of new plants after an area is burned. As shown in the figure, karrikins are similar to one end of the larger strigolactone phytohormones, and mimic the portion that ends up being covalently connected to the receptor. KAI2 is very similar to D14, and is shown here from PDB ID 4jym.