The pros and cons of undeclared work – taking on a part-time job alongside full-time employment – is hotly debated. However, moonlighting is not uncommon in biology, as individual proteins often perform multiple functions. For many years, scientists have known that the Unusual Floral Organ (UFO) protein appears to do moonlighting.
Based on the protein’s structure, its role in plants is believed to be targeting proteins for destruction. But it also works with the Leafy (LFY) protein to support flowering. A team of scientists from France has now elucidated how this protein fulfills two roles.
Flowers and a UFO
When it comes to flowering, the Leafy (LFY) protein is a true workhorse. Flowers are made up of parts called sepals, petals, stamens, and carpels, which are arranged in whorls. The LFY protein, alone or in combination with other proteins, is responsible for activating genes that are essential for the formation of each of these parts. LFY combines with UFO to form petals and stamens.
According to the Studies Lead author, François Parcy from the CNRS and the University of Grenoble Alpes, was the reason why it took more than 25 years to figure out the UFO LFY mechanism, the “misleading nature of the UFO protein”.
UFO belongs to a group of about 700 proteins characterized by a pattern of amino acids called the F-box domain that regulates the levels of other proteins. Parcy said UFO marks other proteins for destruction: “It puts a chemical marker on a protein that has been chosen for destruction. Once a protein is tagged, [some] cell machinery, called [a] Proteasome, recognizes the tag and chops the protein into hundreds of pieces.”
So you might expect UFO to also mark LFY for destruction. “Normally, it should also break down the LFY protein. In the case of LFY, however, we find that the UFO has an entirely different function – that of binding to a region of DNA that LFY alone cannot access,” Parcy said.
When LFY and UFO come together, they attach to DNA near genes essential for the formation of petals and stamens.
Parcy and his team began their research four years ago by making the UFO protein in large quantities in insect cells. “It was quite challenging as the UFO is one of the most difficult proteins to synthesize,” Parcy noted.
Wherever there are flowers
It turns out that UFO doesn’t need to destroy other proteins to work with LFY. “We then modified it by removing the F-box domain, which is responsible for triggering the breakdown of partner proteins. To our surprise, we found that although the protein removed its major function, it still worked well with the LFY protein,” said Parcy. The experiment revealed that the UFO protein serves a different function than just targeting proteins for destruction.
This additional function appears to involve altering the DNA sequences to which Leafy attaches. Researchers obtained the 3D structure of the interaction between LFY, UFO and the regions of DNA to which they bind using cryo-electron microscopy. When UFO and LFY work together, Parcy says they can attach to regions of DNA responsible for forming petals and stamens. None of these proteins can attach to this DNA on their own.
“This means that while each protein has the ability to faintly touch the region of DNA, when combined it contributes to its strength, resulting in an interaction with a new DNA motif,” he said.
The LFY-UFO association is present in all flowering plants. Also in rice, the two proteins LFY and UFO stick together so they can bind new regions of DNA, leading to the development of the part of the plant that holds its grain, called the panicles.
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