UBL/BAG-domain co-chaperones cause cellular stress upon overexpression through constitutive activation of Hsf1

Abstract

As a result of exposure to stress conditions, mutations, or defects during synthesis, cellular proteins are prone to misfold. To cope with such partially denatured proteins, cells mount a regulated transcriptional response involving the Hsf1 transcription factor, which drives the synthesis of molecular chaperones and other stress-relieving proteins. Here, we show that the fission yeast Schizosaccharomyces pombe orthologues of human BAG-1, Bag101, and Bag102, are Hsp70 co-chaperones that associate with 26S proteasomes. Only a subgroup of Hsp70-type chaperones, including Ssa1, Ssa2, and Sks2, binds Bag101 and Bag102 and key residues in the Hsp70 ATPase domains, required for interaction with Bag101 and Bag102, were identified. In humans, BAG-1 overexpression is typically observed in cancers. Overexpression of bag101 and bag102 in fission yeast leads to a strong growth defect caused by triggering Hsp70 to release and activate the Hsf1 transcription factor. Accordingly, the bag101-linked growth defect is alleviated in strains containing a reduced amount of Hsf1 but aggravated in hsp70 deletion strains. In conclusion, we propose that the fission yeast UBL/BAG proteins release Hsf1 from Hsp70, leading to constitutive Hsf1 activation and growth defects.

Gene Expression of Pneumocystis murina after Treatment with Anidulafungin Results in Strong Signals for Sexual Reproduction, Cell Wall Integrity, and Cell Cycle Arrest, Indicating a Requirement for Ascus Formation for Proliferation

“…EMR09987.1 had highest homology to the S. pombe small heat shock protein 16, which was shown to overcome a specific growth suppression in response to viral protein R (Vpr) ( 27 ). EMR09253.1 was homologous to the yeast heat shock protein 70 (Hsp70) family of proteins, specifically Ssa1p of S. pombe , which was recently shown to respond to misfolded proteins during the stress response by targeting them for degradation via the ubiquitin-proteosome system ( 28 ). The Hsp90p chaperone homolog to EMR08493.1 is reported to regulate the activity and stability of about 10% of cellular genes ( 29 ).…”

Co-Chaperones in Targeting and Delivery of Misfolded Proteins to the 26S Proteasome

“…Deletions of the BAG-domain containing NEFs Bag101 and Bag102 in Schizosaccharomyces pombe display no obvious growth phenotypes [ 187 ]. However, overexpression of Bag101, and perhaps other NEF-type co-chaperones, gives rise to a HSF1-mediated stress response that results in a growth defect, and a transcriptional response similar to that observed upon deletion of the HSP70 Ssa2 or the JDP Mas5 (orthologue of yeast Ydj1) [ 188 ]. Since, S. pombe Ssa2 and Mas5 are responsible for binding and inactivation of HSF1 under unstressed conditions [ 189 ], Bag101 therefore likely releases HSF1 from the Ssa1-Mas5 chaperone and initiates the observed transcriptional response [ 188 ].…”

“…Specifically where on the proteasome BAG-1 binds is currently unknown, however, it likely binds subunits also bound by other UBL-domain shuttle factors, such as the Rpn1 (Elsasser et al, 2002) and Rpn2 subunits [ 196 ]. Accordingly, in S. pombe the BAG-1 orthologue Bag101 [ 187 ] competes with the Rad23 orthologue Rhp23 for proteasome binding [ 188 ].…”

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

“…Human BAG-1 interacts with HSP70 via its BAG domain and utilizes the UBL domain in targeting the chaperone cofactor to the 26S proteasome for degradation (Demand et al, 2001). The UBL/BAG domain proteins in S. pombe , SpBAG101 and SpBAG102, display similar interaction pattern to human BAG-1 (Kriegenburg et al, 2014; Poulsen et al, 2017). To validate the functionality of the UBL and the BAG domain of BcBAG1, we generated two truncated forms of BcBAG1; BcBAG1A 1-141 and BcBAG1B 142-298 (Supplementary Figure S1B), containing UBL and BAG domain, respectively.…”

“…Two other BAG1 homologs from Schizosaccharomyces pombe , BAG101 and BAG102 , are co-factors of 26S proteasomes, and play as HSP70 chaperones (Kriegenburg et al, 2014). Overexpression of BAG101 and BAG102 inhibit cell growth by triggering HSP70 to release and activate HSF1 (heat shock factor 1) (Poulsen et al, 2017). To date, the only example in filamentous fungi is BAGA from Aspergillus nidulans , which impacts fungal sexual development and modulates secondary metabolism (Jain et al, 2018).…”