New research revealed a glue-like protein called nucleophosmin, NPM1, binds together the biomolecules inside the nucleolus.

The nucleolus is the largest part of a cell's nucleus. It's a membrane-less structure organized to execute specific functions. The organelle is made up of proteins and ribonucleic acids, or RNA, which fuse in process called liquid-liquid phase separation.

Until now, researchers didn't know the molecular foundation of the nucleolus. Because the nucleolus plays an essential role in building proteins, NPM1s ability to bind with a variety of proteins is especially useful.

"The nucleolus performs a specialized function, and NPM1 seems to have evolved to assist in the process by being able to phase separate with these two important and very different types of nucleolar molecules," Richard Kriwacki, a researcher and structural biologists at St. Jude Children's Research Hospital, said in a press release. "NPM1 is like the glue that holds different factors required for ribosome assembly within the nucleolus."

Previous research has identified NPM1 has an important binding partner for the tumor suppressor protein ARF. More than a third of patients with acute myeloid leukemia have mutated NPM1.

In watching the nucleolus under a microscope and using fluorescent protein tagging, scientists discovered that NPM1's flexible shape allows it to form loose bonds with many different proteins and RNA.

The ability to form loose biomolecular bonds is key to encouraging phase separation and holding on to important proteins and amino acids in the nucleolus.

"There are other proteins in the nucleolus that have some of the same features as NPM1, including the negatively charged amino acid tracts," Kriwacki said. "That suggests that NPM1 is probably not the only protein contributing to phase separation in the nucleolus, but our studies show that it certainly is a very important player."

Researchers published their findings in the journal eLife.