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

Heterotrimeric Gproteins 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, one G, and one G subunit. However, in addition to the canonical G subunits (classA), plants also possess two unusual, plant-specific classes of G subunits (classes B and C) that have not yet been found in animals. These include G subunits lacking the C-terminal CaaX motif (classB), which is important for membrane anchoring of the protein; the presence of such subunits gives rise to a flexible sub-population of G/ heterodimers that are not necessarily restricted to the plasma membrane. Plants also contain classC G subunits, which are twice the size of canonical G 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 AGG3, a classC G 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. Significance Statement AGG3 is the prototype of an unusual G subunit of the heterotrimeric G protein complex. It is a type II membrane protein having a cysteine-rich extracellular domain.