Unterschiedliche Funktionen der TOL-Homologe

Rationale and significance Plants, being sessile organisms, have evolved a plethora of mechanisms to be able to respond quickly and accurately to their surroundings. The plasma membrane (PM), which forms the boundary between the extracellular environment and cellular constituents, is densely packed with a vast array of proteins involved in the sensing and transmitting of stimuli as well as transport processes. The regulation of protein abundance and localization at membranes is achieved by trafficking via the endomembrane system. The TOL (TOM1-like) proteins are responsible for initiating a pathway of guiding ubiquitinated PM proteins to the vacuole for their degradation, functioning as ubiquitin receptors in the early steps of the ESCRT (Endosomal Sorting Complex Required for Transport) pathway in plants [1-3]. Nevertheless, the precise function of the individual TOLs as well as how the TOL homologs and other ubiquitin adaptors are coordinated remains undescribed. There are apparent redundancies within the nine TOL family members, as single knock out plant lines show no obvious phenotype, while a higher order mutant exhibits severe pleiotropic defects. Another higher order TOL mutant, lacking apparent phenotypes under standard conditions, is hypersensitive to the plant hormone abscisic acid (ABA) (section 2.4), signifying that TOLs function discretely in separate pathways. Indications that TOLs perform specific, individual functions comes from their distinct C-terminal domains, their diverse expression patters as well as their different subcellular localization [1-4](section 2.1) and a distinctive sensitivity to drugs disrupting endosomal trafficking (section 2.3). TOLs additionally seem to be subjected to dynamic rearrangements, not only upon certain environmental stresses [1] (section 2.2), but also upon alterations in their post translational modifications [3]. We also observed re-localization of cytoplasmic TOLs to the PM in higher order mutants (section 2.1). In this project, we want to assess how the individual TOLs proteins function in the regulation of the abundance of PM proteins. This is signified by observations, indicating that next to a general role in the endosomal degradation pathway, individual TOLs may exhibit a more discriminating role in distinct signaling pathways, allowing them to fine-tune responses of plants to their ever-changing surroundings.