Abstract
Heterotrimeric G-proteins and the regulator of G-protein signaling (RGS) proteins, which accelerate the inherent GTPase activity of Gα proteins, are common in animals and encoded by large gene families; however, in plants G-protein signaling is thought to be more limited in scope. For example, Arabidopsis thaliana contains one Gα, one Gβ, three Gγ, and one RGS protein. Recent examination of the Glycine max (soybean) genome reveals a larger set of G-protein-related genes and raises the possibility of more intricate G-protein networks than previously observed in plants. Stopped-flow analysis of GTP-binding and GDP/GTP exchange for the four soybean Gα proteins (GmGα1-4) reveals differences in their kinetic properties. The soybean genome encodes two chimeric RGS proteins with an N-terminal seven transmembrane domain and a C-terminal RGS box. Both GmRGS interact with each of the four GmGα and regulate their GTPase activity. The GTPase-accelerating activities of GmRGS1 and -2 differ for each GmGα, suggesting more than one possible rate of the G-protein cycle initiated by each of the Gα proteins. The differential effects of GmRGS1 and GmRGS2 on GmGα1-4 result from a single valine versus alanine difference. The emerging picture suggests complex regulation of the G-protein cycle in soybean and in other plants with expanded G-protein networks.
Highlights
The soybean genome encodes the most expanded plant heterotrimeric G-protein network reported to date
Heterotrimeric G-proteins and the regulator of G-protein signaling (RGS) proteins, which accelerate the inherent GTPase activity of G␣ proteins, are common in animals and encoded by large gene families; in plants G-protein signaling is thought to be more limited in scope
Analysis of GTP Binding and GTP/GDP Exchange in GmG␣1– 4—Initial BODIPY-fluorescence assays suggested that GmG␣1– 4 may have different GTP binding, hydrolysis, and/or exchange kinetics [23]
Summary
The soybean genome encodes the most expanded plant heterotrimeric G-protein network reported to date. AtGPA1 is an extremely slow GTPase with significantly high rates of GTP binding and GDP release and is proposed to exist almost entirely in the GTP-bound conformation Based on these observations, GTP hydrolysis by AtGPA1 is proposed as the rate-limiting step of the Arabidopsis G-protein signaling cycle [28, 31], in contrast to the GDP/GTP exchange of G␣ proteins in mammalian G-protein signaling [2, 4]. The two GmRGS proteins exert differential GAP activities on each of the GmG␣ proteins, and the difference in their GAP activity results from a single valine versus alanine alteration These results suggest a complex regulation of the G-protein cycle in soybean and, by extension, in other plants with expanded G-protein networks
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