University of Cincinnati structural biologists are the first in the world to visualize a key cell protein as part of newly published research from the College of Medicine .
The Seegar Lab has become the first to visualize the structure of a regulator protein, iRhom1, bound to the ADAM17 enzyme, using cryogenic electron microscopy housed in UC's Center for Advanced Structural Biology research facility. This follows the lab's work published last year that visualized the structure of ADAM17 bound to iRhom2.
ADAM17 enzyme activity is essential in humans for proper tissue development and immune response, and regulating its activity is a drug target in treating chronic inflammatory diseases. Ectodomain shedding is the fundamental biological process in which enzymes, such as ADAM17, rapidly cleave and release other protein targets from the cell surface, altering cell-to-cell communication.
So this latest research, published in Cell Reports, identified structural elements in both iRhom1 and iRhom2 that function as a molecular relay, transmitting information across the cell surface and linking intracellular signaling to the activation of ADAM17 enzymes at the cell surface.
"ADAM17 is rapidly activated in response to changes in intracellular signaling networks, yet how these signals are transmitted across the cell membrane to where ADAM17 resides has remained a long-standing question in the field," said corresponding author Tom Seegar, PhD, an assistant professor in the Department of Molecular and Cellular Biosciences and an Ohio Eminent Scholar. The study's co-first authors are Joe Maciag, PhD, a research scientist in the Seegar Lab, and Joe Ungvary, a third-year cancer and cell biology graduate student .
The Seegar Lab also revealed new insights into why the iRhom1 and iRhom2 proteins are considered master regulators of ADAM17, which exists only in a complex with iRhom1 and iRhom2. They found that the structures of both iRhom1 and iRhom2 are identical, as are their responses to intracellular signals, leading to a unified model for enzyme activation.
"While the structures are remarkably similar, their functions are divergent. The ability to maintain distinct roles despite having overall structural similarities can most likely be attributed to the nuance of their sequence, which aids in preferentially recognizing and cleaving substrates," said Maciag.
How they know which function or job to do is unknown, and why they make different decisions is expected to be studied more closely in the future. "It's what's been missing in our field for 30 years," said Seegar.
In addition, the iRhom proteins, particularly iRhom2, will further serve as a novel drug target for treating chronic inflammatory diseases, as they appear to be the drivers of ADAM17 specificity.
Researchers also examined an iRhom1 mutation identified in a patient with cardiomyopathy.
They found the variant was completely defective in supporting iRhom1-ADAM17 function. "We were able to see that iRhom1 proteins were likely not able to fold properly," said Ungvary. "The structure of the protein isn't correct; therefore, its function is null."
In this case, ADAM17 could neither work properly nor reach its target near the cell's surface. Dysregulated ADAM17 activity has been implicated in a wide spectrum of diseases such as chronic inflammation, cancer and neurodegenerative disorders.
"Notably, this phenotype differs from those observed in animal models and may more accurately reflect the consequences of iRhom1 dysfunction in humans," said Seegar. "This is some of the first understanding of how this biology is different in humans and animal models."
Study contributors at UC include: Hala Alnajjar, Conner Slone, Joel Thomas, Maria Rich, Sophia Carazo and Igal Ifergan, PhD, assistant professor in the Department of Molecular and Cellular Biosciences.
Contributors outside UC include: Jose Manuel Perez-Aguilar, PhD; Eliud Morales Dávila, PhD; and Carl Blobel, MD, PhD.
