Phosphovariants of the canonical heterotrimeric Gα protein, GPA1, differentially affect G protein activity and Arabidopsis development

Heterotrimeric G protein signaling downstream of receptor-like kinases (RLKs) is an emerging and important signal transduction mechanism in plants. However, little is known about the effects of phosphorylation events on the function of the canonical G subunit, GPA1. That several known phosphosites reside within important nucleotide co-factor binding sites suggests a role of phosphorylation in modulating the GTP/GDP binding and hydrolysis cycle of GPA1. To mimic and assess the effects of GPA1 phosphorylation, we created ten different phosphovariants of GPA1 and then comprehensively measured in vitro biochemical activity, in vivo protein-protein interactions, and developmental phenotypes of phosphomutant-complemented Arabidopsis gpa1 null mutants. Our assays confirmed both that phosphovariants of S49 and S52 in the G1 nucleotide binding motif impair GTP and GDP binding, and that the gpa1 morphological phenotypes of reduced etiolated hypocotyl elongation, rounded leaves, and short round flowers are not dependent on the nucleotide status of GPA1. Phosphomimetic mutations at S49, S52, T53, and T164 each exhibited a distinct pattern in complementing gpa1 phenotypes, indicating that GPA1 employs multiple signaling states based on phosphorylation status. The multi-state hypothesis provides a key insight into the mechanism by which a limited repertoire of plant heterotrimeric G protein subunits can transduce a wide variety of signals with exquisite specificity.