Tiny protein region unlocks secrets to healthy heartbeat

A tiny region in a little-known muscle protein may hold the key to a healthy, steady heartbeat, as well as possible clues to future treatment of devastating heart ailments.

Washington State University researchers have found a region of a protein called leiomodin that is critical in maintaining the length of tiny filaments that control a person's heartbeat. The work, in collaboration with researchers from the University of Arizona and Mount Sinai School of Medicine in New York, is featured in the high-profile journal Circulation Research.

"It's a small part of a big protein that turned out to be extremely important for its function in the elongation of thin filaments," said Alla Kostyukova, professor in the Gene and Linda Voiland School of Chemical Engineering and Bioengineering who was one of the study's leads.

The heartbeat is a result of intricate, precise coordination among tiny protein filaments in the muscle - so-called thick and thin filaments. Responding to electrical signals, the rope-like filaments bind and unbind, allowing the heart muscle to contract and relax. The thin filaments are made of actin, the most abundant protein in the human body. Two other proteins at the end of actin filaments, tropomodulin and leiomodin, determine the filament length. 

A uniform heart muscle filament length is important at birth for a healthy baby, but the filaments also constantly renew their protein structures throughout a lifetime and need to maintain a consistent length through the years. In families with cardiomyopathy, genetic mutations result in formation of filaments that are either too short or too long. Those affected can have significant heart problems that cause disability, illness and death.

In many cardiomyopathies, the length of the thin filaments is wrong. It always has to be the correct length. If you have a mutation in one of these proteins, your heart cannot work properly."

Alla Kostyukova, professor in the Gene and Linda Voiland School of Chemical Engineering and Bioengineering 

Working for several years to understand the filament regulation, the researchers had determined that there was a competition between leiomodin and tropomodulin to determine the filament length. In this most recent work, the researchers discovered that one region of the leiomodin protein plays a particularly important role in binding to the actin and demonstrated its molecular mechanism.

"For leiomodin, it's a weaker binding, and that's why it was believed that it probably wasn't binding at all," said Kostyukova. "But we demonstrated that it binds forming a so-called 'leaky cap,' and this weaker binding allows it to be removed when the actin starts polymerizing, or building a protein chain. When you make mutations in the binding site to make it even weaker, it makes really long, thin filaments."

As part of the work, the researchers used nuclear magnetic resonance to do a structural analysis of the protein with and without the mutations. Carol Gregorio, their collaborator at Icahn School of Medicine at Mount Sinai, in parallel tested the small region of the protein and verified its behavior in animal cells.

"We created this beautiful result that finally demonstrates for the first time that this region is extremely important for its function as the elongator of thin filaments," said Kostyukova.

The researchers aim to continue gathering information about cardiac proteins. They now understand three of several functional sites and have found the new binding sites. The collaboration between researchers who can conduct animal and cellular studies with a group who is providing the information about the protein structures is indispensable for translating structural insights into biological function.

"These proteins are not well known," she said. "Now we are going to find out how these binding sites work together in this elongation process. Our hope is to get to the point where we can someday work with small molecules to improve this protein when it has pathogenic mutations."

In WSU, in addition to Kostyukova, researcher Garry Smith and Dmitri Tolkachev, assistant professor in the Voiland School, contributed to the study. The work was funded by the National Institutes of Health and the American Heart Association.