Abstract:
Myo-Inositol (Ins)-derived molecules are involved in many cellular processes. Besides the role as compatible solute, Ins represents the building block for a variety of molecules, which function as signaling molecules. Cells utilize insoluble (lipid-bound) and soluble Ins derivatives for diverse signal transduction pathways, many of them essential for cell survival. In yeast and mammalian cells, Ins-derived signaling molecules are involved in many processes, such as phosphate starvation or insulin signaling. In plants first insights in the roles of this class of molecules are just emerging. In this work, I address the roles of Ins-derived signaling molecules during abiotic and biotic stress in Saccharomyces cerevisiae and Arabidopsis thaliana. In the presented manuscript (Johnen et al., in preparation) the role of Ins derivatives in overcoming aluminum (Al) toxicity, representing a major abiotic limitation for crop production worldwide, was investigated. We report that the ectopic expression of AtSFH5 and ScSFH1, encoding proteins, which belong to the ScSec14 lipid transfer protein family, mediates Al tolerance in the model organism yeast, likely independent from the uptake or compartmentalisation of Al. We show that through expression of ScSFH1 the negative charge of the plasma membrane (PM) is increased suggesting PM charge as an early toxicity target for Al. This idea is corroborated by yeast and plant genetics, as well as by cell biological analyses, showing that at physiological relevant Al concentrations proper membrane targeting of proteins is impaired and that an increase or a decrease of the PM charge led to higher or lower Al tolerance in yeast and plants, respectively. Based on a combination of cell biological, genetic and biochemical experiments, we propose a model in which AtSFH5 and ScSfh1 mediate in vivo phospholipid transfer, resulting in either increased levels or increased deprotonation state (and thus activity) of phosphatidyl inositol-4-phosphate (PtdIns(4)P) at the PM, thereby leading to an increase in negative PM charge and Al tolerance. Human fungal pathogens cause invasive fungal infections, which are associated with high morbidity and mortality rates. With only three classes of antifungal drugs in therapeutic use there is a lack of diversity of antifungal agents. In Pries, Nöcker, Khan, Johnen, Hong et al. (2018), we identified two chemotypes, the picolinamides and the benzamides, as specific ScSec14 inhibitors in S. cerevisiae, representing lead structures for the development of antifungal drugs. This finding is based on chemogenomic profiling in yeast and is confirmed by genetic and biochemical evidence. The presented co-crystal structure of ScSec14 bound to a compound belonging to the picolinamide chemotype represents the first structure of inhibitor-bound Sec14 and lays the groundwork for developing new antifungal drugs.
Inositol pyrophosphates (PP-InsPs) are a specific class of Ins derivatives containing one or more high-energy diphosphate (or pyrophosphate) groups. In mammals and yeast these molecules are involved in a variety of cellular processes. In contrast to that, in plants insights in the biosynthesis or the roles of this class of molecules remained elusive at the onset of this work. In Laha et al. (2015) and Laha et al. (2016) we identified two proteins of A. thaliana, AtVIH1 and AtVIH2, as functional PPIP5K homologs catalyzing the biosynthesis of the PP- InsP InsP8 in planta. Based on a series of bioassays investigating the performance of herbivores and fungal pathogens on plant mutant lines defective inositol polyphosphate biosynthesis, we provide evidence that InsP8 plays a role in tuning the Jasmonic acid (JA)- dependent pathogen defense. Furthermore, it is presented that methyl-JA treatment led to an increase of InsP8 and did not have a major effect on other inositol polyphosphate species. Even though mutant lines with decreased InsP8 levels are more susceptible to plant pathogens, the levels of active JA species are increased compared to wild type plants. Based these findings, combined with Ask1-AtCOI1-AtJAZ reconstitution assays, yeast 2-hyrbid (Y2H) studies and in silico docking experiments, we propose that coincidence detection of active JA species and InsP8 by the SCF1AtCOI1-AtJAZ co-receptor complex regulates JA signaling and is thereby regulating JA-related plant defenses. Collectively, the presented data in this work highlight the importance of Ins-derived signaling molecules during abiotic and biotic stress in yeast and plants, and expand our knowledge of the roles of this exciting class of molecules
