Understanding these structural changes will help in studying the effects of known protein mutations in detail and potentially aid in identifying or predicting disease-causing mutations, the researchers say. Heart muscles are a form of striated muscles whose structure helps them perform the fast and continuous pumping movement that requires extreme efficiency. Their contraction and relaxation require energy and it is orchestrated by the interaction of a number of proteins. Genetic mutations causes them to pump either less blood or more blood than necessary.
Such heart (muscle) mutations happen in roughly one in 500 people and the structure itself changes. Sometimes, this could lead to enlarged atria or ventricles and that makes the heart either hyper- or hypo-efficient, says R Sowdhamini, associate professor, NCBS, who co-authored the research paper published in the scientific journal Bioinformatics and Biology Insights earlier this month. The paper's lead author, Margaret Sunitha, is a research student at Sowdhaminis lab, which works on computational approaches to protein science. The other co-authors include James Spudich, a scientist at the Stanford University School of Medicine and John Mercer, visiting professor at the Bangalore-based Institute for Stem Cell Biology and Regenerative Medicine (inStem).
The heart muscle fibre consists of thin and thick filaments composed of actin and myosin, respectively. The actin is a multifunctional protein, while tropomyosin regulates actin mechanics to control muscle contraction, which is a cyclical event spread across three states of protein interaction blocked, closed and open. The latter state represents the maximum myosin binding and force production.
The researchers integrated data from eight different sources, such as electromicroscopy and X-ray crystallography, to demonstrate the interaction of tropomyosin and actin in the open form for the first time. The researchers also divided the sequence of interaction between tropomyosin and actin into seven periods and to study the impact of potential mutations in each period.
We were able to show how the proteins glide on each other and how the movement might happen, says Sowdhamini, adding that structural mutations at key points can impair the system as it moves between the blocked and open states.
These mutations are in important sites where the proteins are supposed to be interacting. If some chemical changes happen at interacting sites, we can imagine how strong the effects could be. We were able to explain the genetic mutations and how it could lead to a disease state from a structural perspective," she says.
The location of these mutations make more sense as a result of the structure shown in this paper, says John Mercer, whose lab at inStem studies mutations in tropomyosin. While the description of these structures is possibly an initial step in the direction towards creating molecules that can treat such mutations, the challenge is also to confirm these structures through further research. These different structures are just probabilities. It's very, very subtle and can be triggered very easily, he says.
So far, there has been uncertainty in understanding the conformational changes responsible for muscle regulation although structures of actin and tropomyosin are available because of the difficulty in catching them interacting through experiments or computer simulations, says Sowdhamini.
The project was supported by the Human Frontier Science Program, an international initiative for research grants, and the NCBS group plans to take its research further by trying to identify potentially vulnerable sites for mutations. We only have a handful of them and in tropomyosin, we only have data on 14 mutations. There may be many more and this would help us to predict the vulnerable sites, she says.