Plants evolved to use a myriad of ubiquitin ligases to regulate their physiological functions. Recent studies on the plant hormone auxin have unveiled a novel paradigm of signal transduction, in which ubiquitin ligases function as hormone receptors. Perceived by the F-box protein subunit of the SCF-TIR1 ubiquitin ligase, auxin directly promotes the recruitment of a family of transcriptional repressors for ubiquitination, thereby activating extensive transcriptional programs.
Our recent structural studies have revealed that auxin functions through a “molecular glue” mechanism to enhance protein-protein interactions with the assistance of another small molecule co-factor inositol hexakisphosphate. Given the extensive repertoire of similar ubiquitin ligases in eukaryotic cells, this novel and widely adopted hormone signaling mechanism in plants may also exist in other organisms.