Developmental control of plant Rho GTPase nano-organization by the lipid phosphatidylserine

Abstract

Rho guanosine triphosphatases (GTPases) are master regulators of cell signaling, but how they are regulated depending on the cellular context is unclear. We found that the phospholipid phosphatidylserine acts as a developmentally controlled lipid rheostat that tunes Rho GTPase signaling in Arabidopsis. Live superresolution single-molecule imaging revealed that the protein Rho of Plants 6 (ROP6) is stabilized by phosphatidylserine into plasma membrane nanodomains, which are required for auxin signaling. Our experiments also revealed that the plasma membrane phosphatidylserine content varies during plant root development and that the level of phosphatidylserine modulates the quantity of ROP6 nanoclusters induced by auxin and hence downstream signaling, including regulation of endocytosis and gravitropism. Our work shows that variations in phosphatidylserine levels are a physiological process that may be leveraged to regulate small GTPase signaling during development.

Auxin-induced nanoclustering of membrane signaling complexes underlies cell polarity establishment in Arabidopsis

“…Our results show that auxin-induced ROP6 acylation and TMK1-modulated lipid ordering are critical for the nanoclustering of the cell surface auxin signaling components. These findings and the complementary work by Jaillais's group showing that auxin induces phosphatidylserinemediated ROP6 nanoclustering in Arabidopsis roots (Platre et al, 2019); altogether, have firmly established a new cell surface auxin signaling mechanism that depends on a localized lipidprotein interaction platform as signaling "hot spot". Interestingly, auxin also promotes the cleavage of intracellular kinase domain from the cell surface TMKs to regulate nuclear gene expression (Cao et al, 2019).…”

“…Taken together, these data strongly support the hypothesis that TMK1 regulates auxin-induced ROP6 nanoclustering via shaping the sterol-rich lipid environment at the PM. Interestingly, the auxin-induced ROP6 nanoclustering has also been described in Arabidopsis roots by Jaillais's group, who showed that phosphatidylserine lipid is important for auxin-mediated ROP6 nanoclustering and phosphatidylserine biosynthesis mutants exhibit severe PC morphogenesis defect (Platre et al, 2019). Thus, ROP6 nanoclusters in PCs may be enriched in both sterols and phosphatidylserines.…”

“…Thus, we propose that the auxin-activated prenylated ROP6 is further modified by acyl (e.g., palmitoyl) groups, which then interact with auxin-induced sterol nanodomains to form more stable ROP6 nanoclusters. Notably, auxin-induced ROP6 nanoclustering also requires phosphatidylserine lipid (negatively charged) and positively charged residues located at the Cterminus of ROP6 in root cells (Platre et al, 2019). Because phosphatidylserine synthesis defects alter PC shape, it is likely phosphatidylserine-dependent ROP6 nanocluster provides another layer of modulation to promote ROP6 nanoclustering.…”

“…Furthermore, neither of these models consider the heterogeneity of the plasma membrane (PM) lipid environment. Increasing evidence shows that many cell surface-signaling proteins are not uniformly mixed with membrane lipids via free-diffusion based equilibrium, but form nanoclusters with specialized lipids (Garcia-Parajo et al, 2014;Harding and Hancock, 2008;Platre et al, 2019;Sezgin et al, 2015;Sezgin et al, 2017;Zhou and Hancock, 2015). Membrane domains enriched in sterols or saturated lipids, often referred to as lipid rafts, have long been suggested as a crucial platform for receptor-mediated signal transduction at the cell surface, particularly in the spatial regulation of key cellular processes, such as cell polarization (Gomez-Mouton et al, 2001;Makushok et al, 2016;Schuck and Simons, 2004;Simons, 2018;Simons and Sampaio, 2011) and immune response (Stone et al, 2017;Varshney et al, 2016;Vieira et al, 2010).…”

“…Membrane domains enriched in sterols or saturated lipids, often referred to as lipid rafts, have long been suggested as a crucial platform for receptor-mediated signal transduction at the cell surface, particularly in the spatial regulation of key cellular processes, such as cell polarization (Gomez-Mouton et al, 2001;Makushok et al, 2016;Schuck and Simons, 2004;Simons, 2018;Simons and Sampaio, 2011) and immune response (Stone et al, 2017;Varshney et al, 2016;Vieira et al, 2010). In addition to sterol-dependent nanodomains, anionic lipids have been implicated in the regulation of nanoclusters of small GTPases, such as Ras, Cdc42 and ROP6 (a Rho-like GTPase from plants), on the PM (Platre et al, 2019;Sartorel et al, 2018;Zhou et al, 2017;. Spatial segregation of signaling molecules into nanoclusters at the PM is a generic phenomenon during signal transduction (Ariotti et al, 2010;Garcia-Parajo et al, 2014;Harding and Hancock, 2008).…”