Though the innate immune system can appear at first glance to be an incomprehensible array of biological interactions, researchers are constantly uncovering nature’s simple solutions to complex immunobiological problems. Two such researchers at Yale, Dr. Ian Berke and Associate Professor of Molecular Biophysics and Biochemistry Yorgo Modis, focus on understanding the mechanisms by which viruses enter and infect the cell, as well as how host cells detect and mount an immune response to the presence of such viruses. Their most recent work, published in The EMBO Journal, has provided insight into the role of the MDA5 protein in these immune responses.
MDA5 is a cytosolic protein that detects viral RNA upon infection by a virus. By binding to RNA, MDA5 initiates a cascade of reactions, including the activation of MAVS, a protein that prompts the release of infection-fighting interferon. However, the exact mechanism by which MDA5 recognizes segments of viral RNA has long been unclear.
A unique characteristic of MDA5 caught the interest of Berke and Modis: it only signals effectively with double-stranded RNA (dsRNA) of at least two kilobases. Whereas most protein binding is dependent on specific chemical structures, MDA5-RNA signaling seems largely to be governed by the physical property of RNA length. This length-sensing property allows the cell to detect the strands of RNA that are often part of a viral genome or a byproduct of infection. By solving the crystal structure of MDA5, Berke and Modis hoped to uncover the biophysical mechanism behind this phenomenon. According to Berke, “It’s like being able to look at the blueprint of an engine to understand how it works — you can design hypotheses based on its structure and test them.”
Originally the team hypothesized a number of complex conformational solutions to the length-sensing problem, but after employing a battery of low-resolution methods, such as electron microscopy and small angle X-ray scattering, what they discovered caught them completely by surprise. “It turns out that nature has thought of a much more elegant and simple solution,” said Modis. Upon placing an MDA5-RNA mixture under a microscope, they observed formation of MDA5 filaments along the dsRNA. The filament formation is a result of cooperative binding which Berke and Modis discovered in MDA5. When one protein binds, it favors binding of the next. They hypothesize that the assembly dynamics of these filaments allow MDA5 to discriminate between long and short RNA; when RNA is too short the filaments are not stable enough to remain intact. This filament formation may also allow MDA5 to interact with MAVS and initiate the interferon response.
Although their hypothesis is consistent with in vivo data, Berke and Modis say their model needs to be tested further. Berke is currently studying filament dynamics to determine the interplay of RNA length and MAVS response levels. Furthermore, this research has a number of public health applications. MDA5 may be a target for future vaccines that take advantage of its ability to prime the adaptive immune system for an effective response.