Identification and characterization of domains regulating auxin transport activity in PIN-FORMED proteins.

• Applicant: Privatdozent Dr. Ulrich Hammes
• Subject Area: Plant Physiology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 547934329

The phytohormone indole acetic acid (IAA), often called "auxin", regulates almost all plant developmental and growth-processes. IAA is polarly transported in plants which regulates the cellular auxin concentration that is important for many auxin dependent processes. At the cellular level, a variety of transport proteins from different protein families are involved in polar auxin transport. PIN-FORMED proteins play a key role in polar-auxin transport. There are two groups of PIN proteins: The long-PINs possess a large, unstructured cytosolic loop, which is necessary for their localization in the plasma membrane as well as activity regulation by kinases, and the short PINs, which lack this loop, are localized in the endoplasmic reticulum and are not subject to regulation by kinases. Recently, the structure of three PINs has been determined and their biochemical and biophysical properties are now better understood. Nevertheless, the disordered loop of PIN proteins, with the exception of the domain containing the kinase target sites, is still-poorly understood. It is therefore unclear how the regulation of activity works mechanistically, which domains within the loop are necessary for the autoinhibition of the proteins, which domains are necessary for activation and which domains confer specific functions. While the elucidation of the structure of the PINs has clarified the transport mechanism and identified some amino acid residues that are important for the function of the transporters, it has not been possible to understand the interaction of the membrane domains with the loop and the physiological significance of the amino acid residues identified as important for transport has not yet been investigated in plants. This is the motivation for this proposal. The experiments focus on PIN2 and PIN3, which have different transport properties, differ in terms of their cell biology and whose respective mutations cause phenotypes that allow-simple complementation studies to be carried out. I would like to dedicate a project to the loop domains and investigate which domains are sufficient and necessary for the regulation of the activity and which mediate specificity. To this end, domains will be deleted or exchanged between PIN2 and PIN3. In the second project, I would like to investigate the interaction between loop domains and membrane domains in order to understand their interplay and characterize the significance of selected amino acid residues for the physiological function of the proteins. The expected results represent a significant advance in the understanding of polar auxin transport and thus plant development and physiology.

• DFG Programme: Research Grants