Posttranslational regulation of plant membrane transporters

The movement of substances across biological membranes is often constrained by physical or energetic barriers, requiring the action of transporter proteins embedded within the lipidic bilayer. These transporters also provide finely tuned regulation of substrate fluxes, essential for maintaining cellular function under both normal and stress conditions. Consequently, transporters are subject to multiple levels of tight regulation, including posttranslational modifications (PTMs). Here, we review the current knowledge on PTMs affecting plant membrane transporters and their impact on protein function. The attachment of chemical groups to protein residues enables rapid modulation of transporter functions, influencing a wide range of protein characteristics. Phosphorylation stands out as the most common PTM, affecting transporter attributes such as activation status, localization and substrate specificity. In turn, ubiquitination acts as a signal for downregulation, either by targeting the transporters for proteasomal degradation or by triggering their endocytosis and subsequent vacuolar sorting. The roles of other, less common PTMs remain unclear, as limited examples exist and recent advances have been sparse. The complex dynamics of substrate transport, which require precise flux magnitudes and directions, appear to demand multi-layered control of the associated transporters. In consequence, further research is needed to investigate individual PTMs affecting transporters, as well as the interplay of multiple PTMs on a single transporter, to better understand how gradual modulation of protein function is achieved.