The Crystal Structure of a Self-Activating G Protein α Subunit Reveals Its Distinct Mechanism of Signal Initiation

In animals, heterotrimeric guanine nucleotide-binding protein (G protein) signaling is initiated by G protein-coupled receptors (GPCRs), which activate G protein a subunits; however, the plant Arabidopsis thaliana lacks canonical GPCRs, and its G protein a subunit (AtGPA1) is self-activating. To investigate how AtGPA1 becomes activated, we determined its crystal structure. AtGPA1 is structurally similar to animal G protein a subunits, but our crystallographic and biophysical studies revealed that it had distinct properties. Notably, the helical domain of AtGPA1 displayed pronounced intrinsic disorder and a tendency to disengage from the Ras domain of the protein. Domain substitution experiments showed that the helical domain of AtGPA1 was necessary for self-activation and sufficient to confer self-activation to an animal G protein a subunit. These findings reveal the structural basis for a mechanism for G protein activation in Arabidopsis that is distinct from the well-established mechanism found in animals.