An Atypical Heterotrimeric G Alpha Protein Has Substantially Reduced Nucleotide Binding But Retains Nucleotide-Independent Interactions With Its Cognate Rgs Protein And G Beta Gamma Dimer

Plants uniquely have a family of proteins called extra-large G proteins (XLG) that share homology in their C-terminal half with the canonical G alpha subunits; we carefully detail here that Arabidopsis XLG2 lacks critical residues requisite for nucleotide binding and hydrolysis which is consistent with our quantitative analyses. Based on microscale thermophoresis, Arabidopsis XLG2 binds GTP gamma S with an affinity 100 times lower than that to canonical G alpha subunits. This means that given the concentration range of guanine nucleotide in plant cells, XLG2 is not likely bound by GTPin vivo. Homology modeling and molecular dynamics simulations provide a plausible mechanism for the poor nucleotide binding affinity of XLG2. Simulations indicate substantially stronger salt bridge networks formed by several key amino-acid residues of AtGPA1 which are either misplaced or missing in XLG2. These residues in AtGPA1 not only maintain the overall shape and integrity of the apoprotein cavity but also increase the frequency of favorable nucleotide-protein interactions in the nucleotide-bound state. Despite this loss of nucleotide dependency, XLG2 binds the RGS domain of AtRGS1 with an affinity similar to the Arabidopsis AtGPA1 in its apo-state and about 2 times lower than AtGPA1 in its transition state. In addition, XLG2 binds the G beta gamma dimer with an affinity similar to that of AtGPA1. XLG2 likely acts as a dominant negative G alpha protein to block G protein signaling. We propose that XLG2, independent of guanine nucleotide binding, regulates the active state of the canonical G protein pathway directly by sequestering G beta gamma and indirectly by promoting heterodimer formation. Communicated by Ramaswamy H. Sarma