Characterization of MYR1, a dosage suppressor of YPT6 and RIC1 deficient mutants

Regulation of plant immune receptor accumulation through translational repression by a glycine-tyrosine-phenylalanine (GYF) domain protein

Plant immunity is tightly regulated to ensure proper defense against surrounding microbial pathogens without triggering autoimmunity, which negatively impacts plant growth and development. Immune receptor levels are intricately controlled by RNA processing and post-translational modification events, such as ubiquitination. It remains unknown whether, and if yes, how, plant immune receptor homeostasis is regulated at the translational level. From a mutant, snc1-enhancing (muse) forward genetic screen, we identified MUSE11/EXA1, which negatively regulates nucleotide-binding leucine-rich repeat (NLR) receptor mediated defence. EXA1 contains an evolutionarily conserved glycine-tyrosine-phenylalanine (GYF) domain that binds proline-rich sequences. Genetic and biochemical analysis revealed that loss of EXA1 leads to heightened NLR accumulation and enhanced resistance against virulent pathogens. EXA1 also associates with eIF4E initiation factors and the ribosome complex, likely contributing to the proper translation of target proteins. In summary, our study reveals a previously unknown mechanism of regulating NLR homeostasis through translational repression by a GYF protein.

Deletion of the sec4 homolog srgA from Aspergillus fumigatus is associated with an impaired stress response, attenuated virulence and phenotypic heterogeneity

Small GTPases of the Rab family are master regulators of membrane trafficking, responsible for coordinating the sorting, packaging and delivery of membrane-bound vesicles to specific sites within eukaryotic cells. The contribution of these proteins to the biology of the human pathogenic fungus Aspergillus fumigatus has not been explored. In this study, we characterized the srgA gene, encoding a Rab GTPase closely related to Sec4. We found that a GFP-SrgA fusion protein accumulated preferentially at hyphal tips and mature condiophores. The radial growth of a ΔsrgA mutant was impaired on both rich and minimal medium, consistent with a role for SrgA in filamentous growth. In addition, the ΔsrgA mutant revealed dysmorphic conidiophores that produced conidia with heterogeneous morphology. The ΔsrgA mutant was hypersensitive to brefeldin A-mediated inhibition of vesicular trafficking and showed increased temperature sensitivity relative to wild type A. fumigatus. However, the most striking phenotype of this mutant was its phenotypic heterogeneity. Individual colonies isolated from the original ΔsrgA mutant showed variable morphology with colony sectoring. In addition, each isolate of the ΔsrgA mutant displayed divergent phenotypes with respect to thermotolerance, in vitro stress response and virulence in a Galleria mellonella infection model. Taken together, these results indicate that SrgA contributes to the asexual development and filamentous growth of A. fumigatus. However, the discordant phenotypes observed among individual isolates of the ΔsrgA mutant suggest that the absence of srgA exerts selective pressure for the acquisition of compensatory changes, such as second-site suppressor mutations.

Notch signaling is antagonized by SAO-1, a novel GYF-domain protein that interacts with the E3 ubiquitin ligase SEL-10 in Caenorhabditis elegans

Notch signaling pathways can be regulated through a variety of cellular mechanisms, and genetically compromised systems provide useful platforms from which to search for the responsible modulators. The Caenorhabditis elegans gene aph-1 encodes a component of γ-secretase, which is essential for Notch signaling events throughout development. By looking for suppressors of the incompletely penetrant aph-1(zu147) mutation, we identify a new gene, sao-1 (suppressor of aph-one), that negatively regulates aph-1(zu147) activity in the early embryo. The sao-1 gene encodes a novel protein that contains a GYF protein-protein interaction domain and interacts specifically with SEL-10, an Fbw7 component of SCF E3 ubiquitin ligases. We demonstrate that the embryonic lethality of aph-1(zu147) mutants can be suppressed by removing sao-1 activity or by mutations that disrupt the SAO-1-SEL-10 protein interaction. Decreased sao-1 activity also influences Notch signaling events when they are compromised at different molecular steps of the pathway, such as at the level of the Notch receptor GLP-1 or the downstream transcription factor LAG-1. Combined analysis of the SAO-1-SEL-10 protein interaction and comparisons of sao-1 and sel-10 genetic interactions suggest a possible role for SAO-1 as an accessory protein that participates with SEL-10 in downregulation of Notch signaling. This work provides the first mutant analysis of a GYF-domain protein in either C. elegans or Drosophila and introduces a new type of Fbw7-interacting protein that acts in a subset of Fbw7 functions.

Isolation of a fission yeast mutant that is sensitive to valproic acid and defective in the gene encoding Ric1, a putative component of Ypt/Rab-specific GEF for Ryh1 GTPase

Valproic acid (VPA) causes various therapeutic and biological effects, but the exact mechanisms underlying these effects, however, remain elusive. To gain insights into the molecular mechanisms of VPA action, we performed in fission yeast a genetic screen for mutants that show VPA hypersensitivity and have identified several membrane-trafficking mutants including vas1-1/vps45 and vas2-1/aps1. Here, we describe the isolation and characterization of vas3-1/ric1-v3, a mutant allele of the ric1 (+) gene encoding a fission yeast homolog of the budding yeast Ric1p, a component of Ypt/Rab-specific guanyl-nucleotide exchange factor (GEF). The Rab GTPase Ryh1 knockout (Deltaryh1) cells and Deltaric1 cells exhibited similar phenotypes. The double knockout Deltaric1Deltaryh1 cells did not display synthetic growth defects. These results are consistent with the notion that Ric1 may be a component of the GEF complex for Ryh1. Overexpression of wild-type Ryh1 and the constitutively active Ryh1Q70L only partially suppressed the phenotypes of ric1-v3 and Deltaric1 cells, and they failed to localize to the Golgi/endosomes in ric1-v3 and Deltaric1 cells. Furthermore, we isolated vps15 (+) gene, encoding a serine/threonine protein kinase, as a dosage-dependent suppressor of the temperature-sensitive phenotype of ric1-v3 mutant, but not that of Deltaric1 cells. Our results showed that the ric1-v3 mutant allele has some residual functional activity and suggest that Vps15 plays a role in the regulation of Ric1 function. In conclusion, Ric1 is a putative component of GEF for Ryh1 and might be regulated by Vps15. Further studies are needed to reveal the mechanism underlying the regulation.