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

The Arabidopsis G protein α subunit exhibits properties necessary and sufficient for receptor-independent activation. Helical Movement Signaling by heterotrimeric guanine nucleotide–binding protein (G protein)–coupled receptors (GPCRs) in animals is a well-studied process. Binding of ligand to the GPCR results in a conformational change that activates the G protein by inducing the exchange of guanosine triphosphate (GTP) for guanosine diphosphate (GDP) at the G protein α subunit. The resulting dissociation of the heterotrimer frees the α and βγ subunits to interact with effector molecules. Arabidopsis lacks the canonical GPCRs of animal cells, and its G protein α subunit, AtGPA1, is self-activating, exhibiting spontaneous nucleotide exchange. Jones et al. solved the crystal structure of AtGPA1 and compared it to that of a mammalian Gαi1. This analysis showed that the helical domain region of AtGPA1 was disordered compared to that of Gαi1, and molecular dynamics simulations demonstrated its dynamic motion. Substitution of the Gαi1 helical domain with that of AtGPA1 made the resulting chimeric protein self-activating, demonstrating that the helical domain of AtGPA1 regulates nucleotide exchange and endows the plant α subunit with a distinct mechanism of activation. In animals, heterotrimeric guanine nucleotide–binding protein (G protein) signaling is initiated by G protein–coupled receptors (GPCRs), which activate G protein α subunits; however, the plant Arabidopsis thaliana lacks canonical GPCRs, and its G protein α subunit (AtGPA1) is self-activating. To investigate how AtGPA1 becomes activated, we determined its crystal structure. AtGPA1 is structurally similar to animal G protein α 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 α 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.