Sorting and Ubiquitylation Control of PIN2 Auxin Carrier in Arabidopsis

Crosstalk between plants and their environment requires tight regulation of information exchange at the plasma membrane (PM). PM proteins mediate such communication, by sensing variations in nutrient availability, external growth determinants and pathogens as well as by controlled solute transport across the membrane border. PIN proteins are membrane proteins essential for inter- and intra-cellular distribution of the growth regulator auxin, influencing numerous aspects of plant development by orchestration of auxin distribution. PIN protein fate has been studied extensively, providing important insights into pathways, controlling plant membrane protein localization and intracellular sorting. Work in my lab focuses on analysis of PIN proteolytic degradation and its biological relevance. Experiments demonstrated that ubiquitylation functions as a signal for endocytic sorting of PIN2 auxin carrier protein, and that variations in PIN ubiquitylation are essential for dynamic adjustments in protein sorting to the vacuole, which in turn appears to modulate adaptive growth responses. However, we are still lacking detailed insights into mechanisms and pathways involved. Our earlier work resulted in identification of cis-acting elements and types of ubiquitylation that control endocytic, degradative sorting of PIN2, and we now aim at the identification of trans-acting factors. By using PIN2 as model protein, we initiated forward and reverse genetic screens to characterize molecular determinants involved in sensing and controlling PIN2 ubiquitylation. We now aim at identification of pathways that mediate communication between plant cells and their environment via turnover of PM proteins, and will study its relevance for the adaptation potential of higher plants.

Plant PIN-FORMED (PIN) proteins represent a key determinant for directional transport of the growth regulator auxin throughout the entire plant body. This is accomplished by their asymmetric polar distribution at the plasma membrane of auxin-transmitting cell files, thereby defining the directionality of auxin transport. PINs are subject to stringent multifaceted regulation, in order to allow for swift and reversible adjustments of auxin flow in response to intrinsic and environmental stimuli. In this project we aimed at the identification of pathways, mediating internalization and proteolytic degradation of the root-specific PIN2 protein in the model plant Arabidopsis thaliana. By employing a constitutively degraded version of PIN2 we identified genetic and signalling determinants, causing stabilization of the otherwise degraded protein. Some of these genetic determinants could be linked to the plants endocytic protein sorting machinery, whereas additional regulators of PIN2 degradation were found to depend on the plant steroid hormone brassinosteroid. Specifically, we observed a crucial role for brassinolide in the reversible stabilization of PIN2 in roots that reorient their growth in response to a gravitational signal. Furthermore, by employing a combination of cell biology and mathematical modelling approaches, we characterized a mechanism, by which plants transmit gravitational signals in the fine-tuning of directional organ growth by means of differential brassinolide signalling and variations therein.