The role of PIN proteins as rate-limiting components directing polar auxin transport is well described. However, not much is known about the determinants and regulators of the polar localization of these membrane proteins. In fact, very little is known about polarity establishment in plant cells, let alone the polar localization of PIN proteins. Experiments by Geldner and coworkers using the exocytotic vesicle trafficking inhibitor Brefeldin A and cytoskeleton depolymerizing drugs indicated that, in interphase cells, PIN1 proteins cycle between the plasma membrane and endosome-like compartments in an actin-dependent and microtubule-independent manner (Geldner et al. 2001). This PIN recycling is likely to be important for both maintaining and facilitating rapid changes in PIN polar localization. However, in dividing cells, PIN deposition was found to occur at both sides of the cell plate in a microtubule-dependent manner (Geldner et al. 2001; Boutte et al. 2006), indicating that polar localization of PIN proteins is established via post-mitotic redistribution. Following their initial observations, Geldner and coworkers identified the ADP-ribosylation factor-GDP/GTP exchange factor (ARF-GEF) EMB30/GNOM as the Brefeldin A sensitive component in the trafficking of PIN1 vesicles from endosomal compartments to the plasma membrane (Geldner et al. 2003). In accordance with this role, emb30/gnom loss-of-function mutants are defective in proper subcellular localization of PIN1 and show developmental defects that resemble those caused by inhibition of polar auxin transport (Steinmann et al. 1999; Geldner et al. 2004). Although the activity of EMB30/GN0M and of other vesicle trafficking components, such as the small GTP binding protein ADP-ribosylation factor 1
(ARF 1) and the ARF-GTPase activating protein (ARF-GAP) SCF/VAN3, has been correlated with polar auxin transport and PIN vesicle trafficking (Xu and Scheres 2005; Sieburth et al. 2006), no clear evidence has yet been provided that these proteins determine PIN polarity.
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