Evidence for an unusual transmembrane configuration of AGG3, a class C Gγ subunit of Arabidopsis

Abstract

SUMMARY Heterotrimeric G proteins are crucial for the perception of external signals and subsequent signal transduction in animal and plant cells. In both model systems, the complex is comprised of one Gα, one Gβ and one Gγ subunit. However, in addition to the canonical Gγ subunits (Class A), plants also possess two unusual, plant-specific classes of Gγ subunits (Classes B and C) not yet found in animals. These include Gγ subunits lacking the C-terminal CaaX motif (Class B) which is important for membrane anchoring of the protein, and thus give rise to a flexible subpopulation of Gβ/γ heterodimers that is not necessarily restricted to the plasma membrane. Even more interesting, plants also contain Class C Gγ subunits which are twice the size of canonical Gγs, with a predicted transmembrane domain, and a large cysteine-rich, extracellular C-terminus. However, neither the presence of the transmembrane domain nor the membrane topology has been unequivocally demonstrated. Here, we provide compelling evidence that AGG3, a Class C Ggamma subunit of Arabidopsis, contains a functional transmembrane domain, which is sufficient but not essential for plasma membrane localization, and that the cysteine-rich C-terminus is extracellular.

Membrane-Localized Extra-Large G Proteins and Gβγ of the Heterotrimeric G Proteins Form Functional Complexes Engaged in Plant Immunity in Arabidopsis

“…However, because similar signals were detected when XLG2 was cotransfected with unfused nYFP, we conclude that XLG2 and AGB1 may not interact directly, but their interaction requires an AGG subunit. This conclusion is in agreement with a recently established fact that AGB1 protein could not be detected in mutant Arabidopsis plants lacking all three AGGs (Wolfenstetter et al, 2014), indicating that it is probably unstable unless engaged with at least one AGG subunit. Importantly, these interactions take place only at the plasma membrane, and not in the nucleus.…”

“…Nevertheless, in all known defense-related functions, AGB1 forms a functional dimer with AGG1 or AGG2 Botella, 2000, 2001;Trusov et al, 2007;Delgado-Cerezo et al, 2012). Moreover, the double mutant agg1 agg2 has reduced steady-state levels of AGB1, while in the triple mutant, lacking all three Gg subunits, AGB1 abundance was even more severely decreased (Wolfenstetter et al, 2014). Therefore, we produced a quadruple mutant agg1 agg2 xlg2 xlg3 to analyze through disease resistance assays.…”

Type B Heterotrimeric G Protein γ-Subunit Regulates Auxin and ABA Signaling in Tomato

“…Type B Gg subunits are similar to type A Gg subunits but lack the C-terminal CaaX motif, precluding the possibility of prenylation . Finally, type C Gg subunits have the conserved domain, a transmembrane domain, and a relatively long Cys-rich C-terminal end Wolfenstetter et al, 2015). A similar classification has been reported for soybean (Glycine max) Gg subunits (Choudhury et al, 2011).…”

HvDep1 Is a Positive Regulator of Culm Elongation and Grain Size in Barley and Impacts Yield in an Environment-Dependent Manner

“…The AGG3-specific COOH-terminal domain was predicted to contain a transmembrane domain followed by a tumor necrosis factor receptor/nerve growth factor receptor (TNFR/NGFR) family type of cysteine-rich signature, as well as four von Willebrand factor type C (VWFC) modules [7]. The initially suggested and recently experimentally proven presence [16] of a transmembrane domain and an extracellular cysteine-rich domain in the COOH-terminus of AGG3 led to the speculation that γ-subunits of the AGG3 type could function as receptors [17]. …”