Cytoprotective Co-chaperone BcBAG1 Is a Component for Fungal Development, Virulence, and Unfolded Protein Response (UPR) of Botrytis cinerea.

Honghong Zhang,Martin B. Dickman,Yurong Li,Zonghua Wang

doi:10.3389/fmicb.2019.00685

Abstract

The Bcl-2 associated athanogene (BAG) family is an evolutionarily conserved group of co-chaperones that confers stress protection against a variety of cellular insults extending from yeasts, plants to humans. Little is known, however, regarding the biological role of BAG proteins in phytopathogenic fungi. Here, we identified the unique BAG gene (BcBAG1) from the necrotrophic fungal pathogen, Botrytis cinerea. BcBAG1 is the homolog of Arabidopsis thaliana AtBAG4, and ectopic expression of BcBAG1 in atbag4 knock-out mutants restores salt tolerance. BcBAG1 deletion mutants (ΔBcbag1) exhibited decreased conidiation, enhanced melanin accumulation and lost the ability to develop sclerotia. Also, BcBAG1 disruption blocked fungal conidial germination and successful penetration, leading to a reduced virulence in host plants. BcBAG1 contains BAG (BD) domain at C-terminus and ubiquitin-like (UBL) domain at N-terminus. Complementation assays indicated that BD can largely restored pathogenicity of ΔBcbag1. Abiotic stress assays showed ΔBcbag1 was more sensitive than the wild-type strain to NaCl, calcofluor white, SDS, tunicamycin, dithiothreitol (DTT), heat and cold stress, suggesting BcBAG1 plays a cytoprotective role during salt stress, cell wall stress, and ER stress. BcBAG1 negatively regulated the expression of BcBIP1, BcIRE1 and the splicing of BcHAC1 mRNA, which are core regulators of unfolded protein response (UPR) during ER stress. Moreover, BcBAG1 interacted with HSP70-type chaperones, BcBIP1 and BcSKS2. In summary, this work demonstrates that BcBAG1 is pleiotropic and not only essential for fungal development, hyphal melanization, and virulence, but also required for response to multiple abiotic stresses and UPR pathway of B. cinerea.

Highlights

Co-chaperones are proteins that assist chaperones in protein folding, oligomeric assembly, and protein transportation and degradation (Hartl, 1996)
We demonstrated that BcBAG1 binds to the endoplasmic reticulum (ER) chaperone BcBIP1 and negatively regulate unfolded protein response (UPR) components, including the expression of BcBIP1, BcIRE1 and the splicing of BcHAC1 mRNA, suggesting BcBAG1 is necessary for the maintenance of the UPR in B. cinerea
Phylogenetic analysis revealed that BcBAG1 shares low similarity with Bcl-2-associated athanogene (BAG) in yeast, plants and animals while it is closely related to BAG homologs from other filamentous fungi, e.g., Sclerotinia sclerotiorum (86.53%), Magnaporthe oryzae (57.04%), Fusarium oxysporum (52.01%), and Aspergillus nidulans (45.30%) (Supplementary Figure S1A and Supplementary Table S3)

Summary

Introduction

Co-chaperones are proteins that assist chaperones in protein folding, oligomeric assembly, and protein transportation and degradation (Hartl, 1996). The Bcl-2-associated athanogene (BAG) family is a group of broadly conserved co-chaperones of 70-kilodalton heat shock protein (HSP70) (Bracher and Verghese, 2015). The BAG family was initially identified by screening mouse cDNA library for Bcl-2 interaction proteins (Takayama et al, 1995). Using the ATPase domain of HSC70/HSP70 as molecular bait in yeast two-hybrid screening, additional BAG family members were identified from human, Caenorhabditis elegans and the fission yeast Schizosaccharomyces pombe (Takayama et al, 1999). BAGs act as scaffolds between HSP70 and target transcription factors or proteins, affecting diverse physiological events (Townsend et al, 2003; Kabbage and Dickman, 2008)

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