Biochemical characterization of the COI1-JAZ receptor for jasmonate

Jasmonates (JAs) are a class of lipid-derived hormones that regulate diverse aspects of plant development and resistance to environmental stress. The molecular mechanism of JA perception is poorly understood. A central component of JA signaling is the F-box protein COI1 that assembles into the E3 ubiquitin ligase SCFCOI1. JAs regulate gene expression by stimulating the ability of SCFCOI1 to degrade JAsmonate ZIM-domain (JAZ) proteins that repress transcriptional activation of JA responsive genes. I employed an in vitro pull-down assay to study the mechanism of COI1-dependent JAZ degradation in response to JA. The results indicate that COI1 physically interacts with JAZ proteins and that this interaction is highly specific for jasmonoyl-isoleucine (JA-Ile) and closely related structures. The bacterial phytotoxin coronatine (COR) stimulated COI1 interaction with tomato JAZ proteins and was at least 100-fold more active than JA-Ile. Testing of a broad range of JA derivatives provided new insight into the structural features of JA-Ile that are required for ligand binding, attenuation of the signal, and the structural basis of COR's enhanced activity. COI1 bears striking sequence and structural similarity to the auxin receptor, TIR1. JA-Ile-dependent binding of COI1 to JAZs is analogous to the role of auxin in promoting the interaction of Aux/IAA proteins with TIR1. Receptor binding studies showed that COI1 is an essential component of a JA receptor. Analysis of truncated JAZs revealed that the C-terminal domain of JAZ3 is necessary and sufficient for ligand-induced COI1-JAZ interaction and ligand binding. Significantly, binding assays performed with purified proteins showed that neither COI1 nor JAZ alone acts as a JA receptor. Rather, COI1 and JAZ together are required for ligand binding. These findings extend the paradigm of F-box proteins as intracellular receptors of small molecules.