BTK autoinhibition analyzed by high-throughput swaps of SH2 domains

Bruton's Tyrosine Kinase (BTK), a Tec-family tyrosine kinase, resembles the Src and Abl kinases in that an SH2-SH3 module regulates the activity of the kinase domain, principally through an inhibitory interaction between the SH3 and kinase domains. In Src-family kinases, phosphorylation of a C-terminal tail latches the SH2 domain onto the kinase domain, stabilizing the inhibitory conformation of the SH3 domain; in Abl, interaction between the kinase domain and a myristoyl group on the N-terminal segment provides a similar latching function. The structure of autoinhibited BTK resembles that of the Src and Abl kinases, but BTK lacks an obvious SH2-kinase latch. To assess the role of the SH2 domain in autoinhibition of BTK, we generated hundreds of chimeric BTK molecules in which the native SH2 domain is replaced by other SH2 domains. We measured the fitness of these chimeric proteins using a high-throughput assay in T and B cells. Surprisingly, many SH2 domains increased fitness when substituted into BTK. Analysis of one set of chimeric proteins indicates that the increase in fitness stems from the ability of the substituted SH2 domains to disrupt BTK autoinhibition while maintaining phosphotyrosine targeting. Thus, although BTK lacks a specialized latch, distributed interactions between the SH2 and kinase domains stabilize the autoinhibitory conformation of BTK. While phosphotyrosine recognition can be conferred on BTK by evolutionarily distant SH2 domains, autoinhibition requires specific interactions with the kinase domain that arose through evolutionary refinement of the regulatory mechanism, and is less easily mimicked by heterologous SH2 domains.