Role of the unfolded protein response in development and pathogenicity of Ustilago maydis
Final Report Abstract
Pathogenic development of the corn smut fungus Ustilago maydis requires the unfolded protein response (UPR) to maintain ER homeostasis during the biotrophic interaction with its host plant maize. The UPR is specifically activated after plant penetration of the fungus and remains constantly active throughout all subsequent stages of pathogenic development. Long-term UPR activity requires modulation of the UPR by the Clp1 protein, which is facilitated by the direct interaction between Clp1 and the Hac1-like UPR regulator Cib1. In this study, we identified a core set of UPR regulated genes that are differentially modulated in their expression by Clp1. We show that expression of Clp1 leads to hyper-resistance towards ER stress and affects the subcellular localization, protein abundance, protein stability and phosphorylation pattern of the Cib1 protein, suggesting that modulated UPR gene expression is connected to these alterations. Within the set of UPR core genes we identified the signal peptide peptidase Spp1 as a novel key factor that is essential for fungal virulence and tumor formation. We show that this function is independent of known SPP substrates and does not involve ERAD mediated protein degradation. Spp1 is not required for vegetative growth, ER stress resistance or pathogenic development before plant penetration but requires its conserved catalytic activity to suppress the plant defense response. Since secretion of effector proteins is not dependent on Spp1 as well, our data indicate that plant defense suppression is either achieved by specific activation of Spp1, or, more likely, by cleavage of substrates that are specifically expressed during the biotrophic stage. In silico prediction of unfolded protein response elements (UPREs) identified 74 effector candidates harboring potential UPREs in their promoters. We tested all 74 genes and verified UPR-dependent expression of the previously known pit2 and tin1-1 effector genes. Cib1 directly regulates expression of these genes and disruption of the regulatory connection between Cib1 and pit1/2 genes resulted in reduced virulence. Moreover, we demonstrate that secretion of Pit2, Pep1 and Tin2 is strictly dependent on the UPR and that processing but not secretion of Cmu1, occurs in an ER-stress dependent manner. In essence, our results provided insight into the modulation of UPR signaling and protein functions to the specific requirements of a highly adapted plant pathogenic fungus.
Publications
- Unfolded Protein Response (UPR) Regulator Cib1 Controls Expression of Genes Encoding Secreted Virulence Factors in Ustilago maydis. PLOS one. 2016;11(4):e0153861
Hampel M., Jakobi M., Schmitz L., Meyer U., Finkernagel F., Doehlemann G., Heimel K.
(See online at https://doi.org/10.1371/journal.pone.0153861) - Signal Peptide Peptidase activity connects the Unfolded Protein Response to Plant Defense Suppression by Ustilago maydis, PLOS Pathogens 15(4): e1007734
Pinter N., Hach CA., Hampel M., Rekhter D., Zienkiewicz K., Feussner I., Poehlein A., Daniel R., Finkernagel F. and Heimel K.
(See online at https://doi.org/10.1371/journal.ppat.1007734)
