Breakthrough study maps hidden protein dynamics in cancer cells

A team of international researchers led by scientists at City of Hope^®, one of the largest and most advanced cancer research and treatment organizations in the United States, with its National Medical Center named a Top 5 "Best Hospital" in the nation for cancer care by U.S. News & World Report, provide the most thorough account yet of an elusive target for cancer treatment. Published today in Science Advances, the study suggests a complex signaling process involving paxillin, a focal adhesion protein that acts as a hub to connect with other proteins, may be vulnerable to therapy despite its fluid state.

Disrupting the interaction of paxillin with focal adhesions bears direct relevance in cancer treatment. This can lead to precision therapeutics targeting a specific paxillin function that is dominant in cancer cells, but less prevalent in healthy cells."

Ravi Salgia, M.D., Ph.D., the Arthur & Rosalie Kaplan Chair in Medical Oncology at City of Hope's comprehensive cancer center

The research adds important new details on a hard-to-characterize network of cellular proteins. Dr. Salgia and his team looked closely at paxillin, which prompts cells to change in response to the environment. This helps cancer cells to evolve and evade detection, while also causing resistance to treatment. Dr. Salgia and his team have been working on elucidating the function of paxillin for over three decades. He and his colleagues were the first to clone the full-length human gene in 1995 at Harvard.

To better understand paxillin's role, the team turned to one of its main binding partners, known as focal adhesion kinase or FAK. The search has proved daunting. These two proteins share a large number of residues needed for binding and are in a constant state of flux. Paxillin is also a heavily disordered protein.

The team narrowed its investigation to characterize only the most relevant structures. Eventually, they found a steady contrast to the disorder. When paxillin and C-terminal targeting domain of FAK (FAT) interact at a specific docking site, they must shrink in size and stay that way to fit a restricted space. Nevertheless, they continue to exert a great deal of flexibility when interacting with the broader focal adhesion network. 

"Our results point to a novel mechanism of protein interaction that is less studied in the literature and indicates the possibility of such mechanisms to be applicable to other disordered proteins," said Dr. Salgia. "This study has broad implications for disordered proteins in general." 

Such protein interactions are typically deemed difficult to control with therapy since there is no clear site for a drug to home in on. But in capturing what they saw, Dr. Salgia and his team were able to construct a model that could help other researchers identify a moving target.

The discovery was made possible by a lot of clever lab work. Utilizing a type of spectroscopy related to medical magnetic resonance imaging (MRI) , that is often employed to study physics, the team was able to better understand the structural characteristics of paxillin. The team then combined spectroscopy with dynamic simulations to show how paxillin binds to FAT. Finally, the team created a computer 3D model to demonstrate how this interaction plays out.

"The combination of all these methods enabled us to accurately characterize the structural features of the paxillin-FAK interaction more than any single method on its own," said Supriyo Bhattacharya, Ph.D., assistant research professor in the Department of Computational and Quantitative Medicine at City of Hope, the first and co-senior author and lead in protein structure and data analysis in the study. 

In addition to Dr. Salgia and City of Hope authors Bhattacharya and Prakash Kulkarni, Ph.D., a research professor in the Department of Medical Oncology & Therapeutics Research at City of Hope (an expert in disordered protein), the team included researchers from the University of Maryland, John Orban, Ph.D., (co-senior author and expert in spectroscopic techniques), and the National Institute of Standards and Technology.