Structure of the CABIT2 domain of THEMIS reveals a novel protein fold with an inserted SH3-like domain

Maturation of thymocytes into T cells is critical for proper function of the adaptive immune system. During this developmental process, thymocytes undergo a highly-regulated selection process regulated by the signaling characteristics of the T cell receptor (TCR) pathway. Thymocyte-Expressed Molecule Involved in Selection (THEMIS) is an essential protein for T cell development. THEMIS regulates phosphatases downstream of the T cell receptor to ensure signaling thresholds are met during selection. Important features of THEMIS are its two uncharacterized CABIT (Cysteine-containing All-Beta In THEMIS) domains, which are intriguing because they have been proposed to participate in important protein-protein interactions (PPIs) that modulate immunological signals. Here, we report the 2.9 Å crystal structure of the THEMIS CABIT2 domain determined via heavy atom phasing. The structure revealed a novel protein domain fold comprised mainly of β-sheets with two distinct subdomains. This domain appears to have a different C-terminal boundary than predicted or found in previously used experimental constructs. Inclusion of the proline rich segment enables GRB2 to bind CABIT2. Isolated CABIT2 domain is unable to bind or modulate the function of SHP1 phosphatase. This structure will provide the foundation for future structure-function studies of CABIT domains and THEMIS.