Sterol-dependent endocytosis mediates post-cytokinetic acquisition of PIN2 auxin efflux carrier polarity

Abstract

The polarization of yeast and animal cells relies on membrane sterols for polar targeting of proteins to the plasma membrane, their polar endocytic recycling and restricted lateral diffusion. However, little is known about sterol function in plant-cell polarity. Directional root growth along the gravity vector requires polar transport of the plant hormone auxin. In Arabidopsis, asymmetric plasma membrane localization of the PIN-FORMED2 (PIN2) auxin transporter directs root gravitropism. Although the composition of membrane sterols influences gravitropism and localization of two other PIN proteins, it remains unknown how sterols contribute mechanistically to PIN polarity. Here, we show that correct membrane sterol composition is essential for the acquisition of PIN2 polarity. Polar PIN2 localization is defective in the sterol-biosynthesis mutant cyclopropylsterol isomerase1-1 (cpi1-1) which displays altered sterol composition, PIN2 endocytosis, and root gravitropism. At the end of cytokinesis, PIN2 localizes initially to both newly formed membranes but subsequently disappears from one. By contrast, PIN2 frequently remains at both daughter membranes in endocytosis-defective cpi1-1 cells. Hence, sterol composition affects post-cytokinetic acquisition of PIN2 polarity by endocytosis, suggesting a mechanism for sterol action on establishment of asymmetric protein localization.

Vesicle Trafficking during Somatic Cytokinesis

“…This has been attributed to the formation of a pool of cell plate-forming vesicles before the onset of cytokinesis and was used to argue in favor of the contribution of endocytic vesicles to cell plate formation (Yasuhara et al, 1995;Dhonukshe et al, 2006). BFA sensitivity in BY-2 cells implicates ARF-dependent membrane trafficking, but not exclusively Golgiderived vesicle trafficking, as the target of BFA in this system is not known (Schrick et al, 2004;Men et al, 2008). The nature and presence of the cell plateforming vesicles remain to be addressed in BY-2 cells, but in Arabidopsis the RAB-A2/A3 compartment has been proposed to fulfill this function.…”

“…This is embodied by the initial accumulation of the PIN2 protein at either side of the cell plate during the early phases of cell plate formation and endocytosis of PIN2 from the basal side of the cell once the cell plate is constructed. Side-specific removal of PIN2 from the basal PM is controlled by the sterol composition of the membrane, as the sterol biosynthesis mutant cyclopropylsterol isomerase1-1 (cpi1-1) fails to remove PIN2 from the basal side of the fused cell plate following cytokinesis (Boutte et al, 2006;Men et al, 2008). Next to failure in establishing PIN2 polarity, cpi1-1 shows cytokinesis defects, which are also reported for several other sterol biosynthesis mutants like fackel ( fk), hydra1 (hyd1), and sterol methyltransferase1/cephalopod (smt1/cph; Schrick et al, 2004;Men et al, 2008).…”

“…Side-specific removal of PIN2 from the basal PM is controlled by the sterol composition of the membrane, as the sterol biosynthesis mutant cyclopropylsterol isomerase1-1 (cpi1-1) fails to remove PIN2 from the basal side of the fused cell plate following cytokinesis (Boutte et al, 2006;Men et al, 2008). Next to failure in establishing PIN2 polarity, cpi1-1 shows cytokinesis defects, which are also reported for several other sterol biosynthesis mutants like fackel ( fk), hydra1 (hyd1), and sterol methyltransferase1/cephalopod (smt1/cph; Schrick et al, 2004;Men et al, 2008). For fk, hyd1, and smt1/ cph, these defects are likely caused by defective cell plate maturation, as these mutants contain reduced levels of cellulose, together with ectopic callose and lignin deposits (Schrick et al, 2004).…”

Arabidopsis phosphatidylinositol monophosphate 5-kinase 2 is involved in root gravitropism through regulation of polar auxin transport by affecting the cycling of PIN proteins

“…Besides the fact that PIP5K2 regulates the PIN protein cycling through PtdIns(4,5)P 2 -mediated vesicle trafficking process, another possibility is that PIP5K2 deficiency and consequent insufficiency of PtdIns(4,5)P 2 may cause changes of the whole-membrane lipid composition, some of which are known to be critical for PIN cycling [47,48]. Although the possible roles of Arabidopsis PIPKs in vesicle regulation have been indicated [16-18, 20, 22], the downstream targets are not well characterized yet.…”

“…This process is dependent on clathrin-mediated endocytosis, intracellular vesicle trafficking [44,45] and actin cytoskeleton, and is controlled by auxin signaling and membrane sterols [12,[46][47][48].…”