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

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 comprises one G α, one G β, and one G γ subunit. However, in addition to the canonical G γ subunits (class  A ), plants also possess two unusual, plant‐specific classes of G γ subunits (classes B and C ) that have not yet been found in animals. These include G γ subunits lacking the C –terminal C aa X motif (class  B ), which is important for membrane anchoring of the protein; the presence of such subunits gives rise to a flexible sub‐population of G β/γ heterodimers that are not necessarily restricted to the plasma membrane. Plants also contain class  C G γ subunits, which are twice the size of canonical G γ subunits, 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 have been unequivocally demonstrated. Here, we provide compelling evidence that AGG 3, a class  C G γ subunit of A rabidopsis, contains a functional transmembrane domain, which is sufficient but not essential for plasma membrane localization, and that the cysteine‐rich C –terminus is extracellular.