PIN2 Ubiquitination and Proteolytic Turnover

PIN proteins represent a plant-specific family of membrane proteins that mediate directional transport of the plant growth regulator auxin, thereby affecting a multitude of developmental processes and growth responses. Polarity of auxin transport seemingly depends on the asymmetric localization of PIN proteins at the plasma membrane, and, consistent with these findings, regulators of sub-cellular PIN localization were found to influence directionality of auxin transport. Now, additional findings demonstrate that a cell-specific control of PIN steady state protein levels might participate in the orchestration of auxin-mediated growth responses as well. Specifically, control of PIN2 protein levels in roots that respond to a gravity stimulus appears to affect the distribution of auxin, thereby controlling bending of the root tip according to the gravity vector. Control of PIN2 degradation depends on the impact of gravity stimulation and variations in auxin concentrations, which combine to control the fate of the protein. Moreover, ubiquitination, which serves as a common signal for membrane protein internalization and degradation in non-plant organisms, could be demonstrated for PIN2. Thus, ubiquitination of plasma membrane proteins such as PIN2 could play a role in the control of membrane protein fate in plants. In this project, we intend to study the consequences of plant plasma membrane protein ubiquitination in general and, by using PIN2 as a model protein, its regulatory implications for auxin distribution and growth responses in particular. Localization studies together with targeted mutagenesis of PIN2 should reveal cis-acting determinants that control PIN2 ubiquitination and consequences of ubiquitination on PIN2 distribution and degradation. Moreover, it is planned to characterize a number of candidate loci that appear to control PIN2 protein levels and expression pattern in trans. Overall, suggested work should provide us with a framework of regulatory events that integrate intrinsic and external signals and the control of developmental processes via regulation of PIN protein levels.

Plant development is characterized by a remarkable flexibility in the control of development and adaptive growth responses. These characteristics represent a major difference between plants and animals that exhibit pre- determined pattern formation and development. A tightly coordinated distribution of the plant growth regulator auxin within the plant body, is mediated by families of transport proteins, and is crucial for dynamic plant growth responses. In this project we analyzed mechanisms that control levels of auxin carrier proteins and hence impact on the amounts of auxin that are distributed within cells and tissues. Modification of an auxin transport protein by another protein, known as ubiquitin, functions as a signal triggering removal of the auxin carrier from the cell boundary into intracellular compartments, resulting in degradation of the protein. Within this project we were able to identify key aspects of this destructive pathway and found evidence for its function in the control of root growth. Apart from regulation of protein turnover, we identified key regulators of translational control (i.e. regulators that are involved in protein translation), with a profound impact specifically on auxin transport and signaling. This highlights a scenario in which defects in a highly general aspect of cell functionality result in very defined growth aberrations, underlining the central role of auxin for plant development.