Evolution of A bHLH Interaction Motif.

Intrinsically disordered proteins and regions with their associated short linear motifs play key roles in transcriptional regulation. The disordered MYC-interaction motif (MIM) mediates interactions between MYC and MYB transcription factors in that are critical for constitutive and induced glucosinolate (GLS) biosynthesis. GLSs comprise a class of plant defense compounds that evolved in the ancestor of the Brassicales order. We used a diverse set of search strategies to discover additional occurrences of the MIM in other proteins and in other organisms and evaluate the findings by means of structural predictions, interaction assays, and biophysical experiments. Our search revealed numerous MIM instances spread throughout the angiosperm lineage. Experiments verify that several of the newly discovered MIM-containing proteins interact with MYC TFs. Only hits found within the same transcription factor family and having similar characteristics could be validated, indicating that structural predictions and sequence similarity are good indicators of whether the presence of a MIM mediates interaction. The experimentally validated MIMs are found in organisms outside the Brassicales order, showing that MIM function is broader than regulating GLS biosynthesis.