Structural characterization of TIR-domain signalosomes through a combination of structural biology approaches

The TIR (Toll/interleukin-1 receptor) domains are found in proteins with roles in the immune systems of humans, plants and bacteria. A combination of structural methods ranging from X-ray and electron crystallography to cryogenic electron microscopy and nuclear magnetic resonance spectroscopy has been required to understand how these domains contribute to signalling, highlighting the complementarity of different structural approaches.

The TIR (Toll/interleukin-1 receptor) domain represents a vital structural element shared by proteins with roles in immunity signalling pathways across phyla (from humans and plants to bacteria). Decades of research have finally led to identifying the key features of the molecular basis of signalling by these domains, including the formation of open-ended (filamentous) assemblies (responsible for the signalling by cooperative assembly formation mechanism, SCAF) and enzymatic activities involving the cleavage of nucleotides. We present a historical perspective of the research that led to this understanding, highlighting the roles that different structural methods played in this process: X-ray crystallography (including serial crystallography), microED (micro-crystal electron diffraction), NMR (nuclear magnetic resonance) spectroscopy and cryo-EM (cryogenic electron microscopy) involving helical reconstruction and single-particle analysis. This perspective emphasizes the complementarity of different structural approaches.