N-terminal domains of DELLA proteins are intrinsically unstructured in the absence of interaction with GID1/gibberellic acid receptors.

The plant growth-repressing DELLA proteins (DELLAs) are known to represent a convergence point in integration of multiple developmental and environmental signals in planta, one of which is hormone gibberellic acid (GA). Binding of the liganded GA receptor (GID1/GA) to the N-terminal domain of DELLAs is required for GA-induced degradation of DELLAs via the ubiquitin-proteasome pathway, thus derepressing plant growth. However, the conformational changes of DELLAs upon binding to GID1/GA, which are the key to understanding the precise mechanism of GID1/GA-mediated degradation of DELLAs, remain unclear. Using biophysical, biochemical, and bioinformatics approaches, we demonstrated for the first time that the unbound N-terminal domains of DELLAs are intrinsically unstructured proteins under physiological conditions. Within the intrinsically disordered N-terminal domain of DELLAs, we have identified several molecular recognition features, sequences known to undergo disorder-to-order transitions upon binding to interacting proteins in intrinsically unstructured proteins. In accordance with the molecular recognition feature analyses, we have observed the binding-induced folding of N-terminal domains of DELLAs upon interaction with AtGID1/GA. Our results also indicate that DELLA proteins can be divided into two subgroups in terms of their molecular compactness and their interactions with monoclonal antibodies.