This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Most organisms rely on an innate immune system as their first line of defense against infection. Within the innate immune system, the Toll-like receptors (TLRs), a family of evolutionarily ancient receptors found on the surface of many cell types, are critical for pathogen recognition outside the cell. About 12 TLRs recognize structures specific to pathogens, such as bacterial cell wall components, bacterial filament proteins, or certain types of nucleic acid. This recognition event initiates a signal inside the cell, which induces the rapid secretion of antimicrobial and inflammatory proteins. Inside the cell, the NOD proteins and RNA helicases such as MDA5 recognize similar pathogen-associated structures to those recognized by TLRs. Remarkably, given the structural diversity of the structures that they recognize, all TLRs and NODs rely on a "leucine-rich repeat" (LRR) domain to recognize pathogen-associated structures. The overall goal of our research program on innate immune sensors is to understand how they recognize conserved molecular patterns in pathogens, and how this recognition is translated into an innate immune response. Our structural approach will provide unique insights into these important processes. First, we aim to determine the structure of one or more TLR-ligand complexes, by X-ray crystallography. Alternative crystallization targets are NOD-ligand or helicase-RNA complexes. We propose to use novel protein expression techniques to maximize protein yields. Our structures will likely define novel principles of molecular recognition. By revealing the conformational changes associated with ligand binding, the structures will provide insight on how pathogen recognition is translated into a signal in the cell that elicits an immune response. Our work will also guide efforts to design synthetic agonists or antagonists with immunomodulatory properties. Such compounds would have a wide range of medical applications, particularly as vaccine adjuvants or anti-inflammatory therapeutics.