Silencing is golden: Scientists image molecules vital for gene regulation

Scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have been studying the molecules that act at the genetic level to give rise to different types of cells. Some of these molecules are a complex of proteins called the Polycomb Repressive Complex 2 (PRC2) that is involved in "silencing" genes so that they are not "read" by the cellular machinery that decodes genetic information, effectively keeping the genetic information in the "off" state.

In two new studies, a team of researchers led by Eva Nogales, senior faculty scientist in the Molecular Biophysics and Integrated Bioimaging (MBIB) Division, has gained insight into the structure of PRC2 and the ways in which it is regulated to affect gene silencing. Their work was reported on January 18 in the journal Science and on January 29 in Nature Structural and Molecular Biology by Eva Nogales and postdoctoral researchers Vignesh Kasinath and Simon Poepsel.

Both publications provide a structural framework to understand PRC2 function, and in the case of the latter, the structures are the first to illustrate how a molecule of this type engages with its substrate. The structural descriptions of human PRC2 with its natural partners in the cell lend important insight into the mechanism by which the PRC2 complex regulates gene expression. This information could provide new possibilities for the development of therapies for cancer.

PRC2 is a gene regulator that is vital for normal development. Genomic DNA is packaged into nucleosomes, which are formed by histone proteins that have DNA wrapped around them. Histone proteins have long polypeptide tails that can be modified by the addition and removal of small chemical groups. These modifications influence the interaction of nucleosomes with each other and other protein complexes in the nucleus. The function of PRC2 in the cell is to make a particular chemical change in one of the histones. The genes in the regions of the genome that have been modified by PRC2 are switched off, or become silenced.

"Not surprisingly, elaborate mechanisms have evolved to ensure that PRC2 marks the correct regions for silencing at the right time," said Nogales, who is also a Howard Hughes Medical Investigator and professor of Biochemistry, Biophysics and Structural Biology at the University of California, Berkeley. Failure of this regulation not only impairs the process of development, but also contributes to the reversal of cell differentiation and the uncontrolled cell growth that are the hallmarks of cancer. "Therefore," Nogales continued, "gaining insight into how PRC2 function is adjusted both in space and time is crucial to understanding cell development."

Nogales and her team use structural biology to elucidate how biomolecules, particularly proteins and nucleic acids (DNA, RNA), are organized and combine to form functional biological assemblies. Obtaining detailed insights into their three-dimensional shape will not only help to understand how they function but also how this function is regulated in the cell. These two studies rely on cryo-electron microscopy for imaging the biomolecules, a technique that can see large biomolecules on a very small scale and in multiple conformations. Kasinath and Poepsel, have now solved the structure of PRC2, which provides a framework to understand how this complex is regulated to modify histone proteins.

The first study, published January 18 in Science by Kasinath, Poepsel, Nogales, and coworkers, visualized the architecture of the complete PRC2 in atomic detail. First author Vignesh Kasinath said, "It took three years of work to obtain this high-resolution structure of all the parts, or subunits, that make up a functional PRC2, as well as visualize how additional protein subunits, called cofactors, may help regulate its activity. Remarkably, both cofactors mimic the histone protein tail in their binding to PRC2 suggesting that cofactors and histone tails together work hand-in-hand to regulate PRC2 function. This structural work holds great promise for new drug development to fight PRC2 dysfunction in cancer."

This work is complemented by a second study that presents snapshots of PRC2 binding to the histone proteins that it modifies as a signal for gene silencing. The structures, which have been published in Nature Structural and Molecular Biology on January 29 by Poepsel, Kasinath and Nogales this week, illustrate beautifully the action of this sophisticated complex. "PRC2 can simultaneously engage two nucleosomes," said Poepsel, first author of this study. "Our cryo-EM images help us understand how the complex can recognize the presence of a histone modification in one nucleosome and place the same tag onto a neighboring nucleosome." This cascade of activity enables PRC2 to spread this modification over the entire neighboring gene loci, thereby marking it for silencing. Nogales added, "The visualization of such interactions is notoriously hard. We have made an important step forward in our general understanding of how gene regulators can bind to and recognize nucleosomes."

PRC2 is essential to gene regulation and expression in all multicellular organisms. The findings from both studies open up tremendous possibilities for combatting cancer while simultaneously expanding our knowledge of gene regulation at a molecular level. "Because PRC2 is deregulated in cancers, it makes a good target for potential therapeutics," said Nogales. The fundamental understanding of PRC2 arising from these studies will have broad implications in both plant and animal biology.

This work was funded by the Howard Hughes Medical Institute and Eli Lilly. This research used cryo-electron microscopy (cryo-EM) and made use of the unique resources of the Bay Area Cryo-EM Facility. Image analysis relied on heavy computational work that was carried out at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility. Vignesh Kasinath was supported by a postdoctoral fellowship from Helen Hay Whitney and Simon Poepsel was supported by the Alexander von Humboldt foundation (Germany) as a Feodor-Lynen postdoctoral fellow.