Themis and Grb2 form a constitutive structural hub in T cell receptor signalling
Positive selection of thymocytes is essential for laying the foundations of the mammalian immune system that include the T cell repertoire, self-tolerance, and prevention of autoimmunity. Themis, the archetypal member of a metazoan protein family featuring distinctive CABIT domains, crucially regulates thymocyte positive selection by linking signalling by the T cell receptor (TCR) to the linker of activation of TCR (LAT). Intriguingly, Themis has been proposed to function via a constitutive complex with the multifunctional adaptor Grb2. Although poised to represent a paradigm shift in our understanding of TCR signalling, the structural and mechanistic basis of such an assembly has remained enigmatic. Here, we present the cryo-EM structure of Themis in complex with Grb2, which reveals how the tandem CABIT domains of Themis engulf the C-terminal SH3 domain of Grb2 (Grb2SH3C) to enable its latching onto the proline rich sequence of Themis. The remaining two domains of Grb2 adopt at least three conformational poses set to interact with other binding partners such as Sos1. Structural insights from unbound Themis unmask the pronounced flexibility of the CABIT domains of Themis, which becomes ordered upon binding to Grb2 to create a binding hotspot for their constitutive complex. Indeed, Themis variants that abrogate interactions with Grb2 also fail to activate the tyrosine phosphatase SHP-1 after TCR stimulation, analogous to the functional phenotype of Themis-deficient cells. Collectively, our study draws the blueprint of the Themis-Grb2 complex as a dynamic structural hub in T cell development.