Saltational evolution of the heterotrimeric G protein signaling mechanisms in the plant kingdom

Signaling proteins evolved diverse interactions to provide specificity for distinct stimuli. Signaling complexity in the G protein (heterotrimeric guanosine triphosphate-binding protein) network was achieved in animals through subunit duplication and incremental evolution. By combining comprehensive and quantitative phenotypic profiles of Arabidopsis thaliana with protein evolution informatics, we found that plant heterotrimeric G protein machinery evolved by a saltational (jumping) process. Sequence similarity scores mapped onto tertiary structures, and biochemical validation showed that the extra-large G alpha (XLG) subunit evolved extensively in the charophycean algae (an aquatic green plant) by gene duplication and gene fusion. In terrestrial plants, further evolution uncoupled XLG from its negative regulator, regulator of G protein signaling, but preserved an a-helix region that enables interaction with its partner G beta gamma. The ancestral gene evolved slowly due to themolecular constraints imposed by the need for the protein tomaintain interactions with various partners, whereas the genes encoding XLG proteins evolved rapidly to produce three highly divergent members. Analysis of A. thaliana mutants indicated that these G alpha and XLG proteins all function with G beta gamma and evolved to operate both independently and cooperatively. The XLG-G beta gamma machinery specialized in environmental stress responses, whereas the canonical G alpha-G beta gamma retained developmental roles. Some developmental processes, such as shoot development, involve both Ga and XLG acting cooperatively or antagonistically. These extensive and rapid evolutionary changes in XLG structure compared to those of the canonical G alpha subunit contrast with the accepted notion of how pathway diversification occurs through gene duplication with subsequent incremental coevolution of residues among interacting proteins.