Nanobodies identify an activated state of the TRIB2 pseudokinase

Tribbles proteins (TRIB1–3) are a pseudokinase-only branch of the human kinome, which recruit substrates to the COP1 ubiquitin-ligase for ubiquitination. TRIB2 was the first Tribbles ortholog to be implicated as a myeloid leukaemia oncogene, by way of recruiting the C/EBPa transcription factor for degradation by COP1. Here we report selection and characterisation of nanobodies against the TRIB2 pseudokinase domain from a synthetic yeast surface-display library. We identified nanobodies that bind the TRIB2 pseudokinase domain with low nanomolar affinity. A crystal structure of Nb4.103 in complex with TRIB2 identified a mode of binding to the N-terminal lobe of the pseudokinase, in a manner that enables specific recognition of TRIB2 over TRIB1 and TRIB3. In the nanobody-stabilised state, TRIB2 adopts an activated conformation that is remarkably similar to the C/EBPa-bound state of TRIB1. Characterization in solution revealed that Nb4.103 can stabilise a TRIB2 pseudokinase domain dimer in a face-to-face manner. Conversely, a distinct nanobody (Nb4.101) binds through a similar epitope but does not readily promote dimerization. In combination, this study identifies features of TRIB2 that could be exploited for the development of inhibitors, and nanobody tools for future investigation of TRIB2 function.