Heterotrimeric G-protein signaling is involved in numerous processes in plants, including germination, cell division, and stress responses, yet few specific downstream targets have been identified. Huang et al. (pages 1226–1238) show that the plastid protein THYLAKOID FORMATION1 (THF1) interacts with GPA1, the Gα subunit of the heterotrimeric G-protein complex in Arabidopsis, as part of a sugar signaling mechanism between plastids and the plasma membrane that influences root growth.
THF1 and GPA1 were found to interact in yeast two-hybrid tests and in vitro and in vivo coimmunoprecipation experiments. Using Förster resonance energy transfer (FRET) analysis, the authors show that the interaction between THF1, which is localized to the stroma, outer plastid membrane, and plastid stromules, and GPA1, which is localized at the plasma membrane, occurs at sites where the plastid membrane abuts the plasma membrane.
This work is significant because it uncovers a mechanism for communication between plastids and the plasma membrane, identifies a previously undiscovered role for G-proteins in sugar signaling, and provides support for the hypothesis that stromules (tubular extensions of plastids that have an undetermined function) play a role in signaling. In addition, it is an excellent example of the use of FRET to investigate in vivo protein–protein interactions in plants.
Figure .
THF1-GFP fusion protein localizes to plastid stromules that appear to tether the plastid to the plasma membrane.