PIN-FORMED (PIN)-dependent auxin transport is essential for many aspects of plant development, and PIN proteins show a polar localisation within cells that determines the direction of auxin flow. Here, Masao Tasaka and co-workers show that Arabidopsis MACCHI-BOU 4 (MAB4)-like proteins can specifically retain polarised PIN proteins at the plasma membrane of cells and, hence, can control directional auxin transport and plant development (p. ). Using genetic anaylsis, the researchers show that combined mutations of MAB4 subfamily members result in auxin-related phenotypes and severe reductions in PIN polarisation. Furthermore, they report, mutations of MAB4 genes result in increased PIN internalisation from the plasma membrane but have no effect on intracellular trafficking. Notably, they show that the MAB4 subfamily proteins are localised to the cell periphery in a polarised manner almost identical to that of PIN localisation. These findings suggest that MAB4 proteins, which are related to NON-PHOTOTROPHIC HYPOCOTYL 3 (NPH3) proteins, may regulate the specific localisation of PIN proteins to the plasma membrane.
