A direct regulatory interaction between chaperonin TRiC and stress-responsive transcription factor HSF1

Abstract

Heat shock transcription factor 1 (HSF1) is an evolutionarily conserved transcription factor that protects cells from protein-misfolding-induced stress and apoptosis. The mechanisms by which cytosolic protein misfolding leads to HSF1 activation have not been elucidated. Here, we demonstrate that HSF1 is directly regulated by TRiC/CCT, a central ATP-dependent chaperonin complex that folds cytosolic proteins. A small-molecule activator of HSF1, HSF1A, protects cells from stress-induced apoptosis, binds TRiC subunits in vivo and in vitro, and inhibits TRiC activity without perturbation of ATP hydrolysis. Genetic inactivation or depletion of the TRiC complex results in human HSF1 activation, and HSF1A inhibits the direct interaction between purified TRiC and HSF1 in vitro. These results demonstrate a direct regulatory interaction between the cytosolic chaperone machine and a critical transcription factor that protects cells from proteotoxicity, providing a mechanistic basis for signaling perturbations in protein folding to a stress-protective transcription factor.

Figure 1. HSF1A protects cells from stress-induced apoptosis

(A) NIH3T3 cells were pretreated with 50 μM HSF1A for 15 h followed by 0.4 ug/ml tunicamycin for 15 h. (B) Total protein was isolated from NIH3T3 cells treated as in (A) and analyzed for Hsp70, Bip, Erdj3 expression and caspase-3 cleavage, with GAPDH as loading control, by immunoblotting. (C) INS 832/13 cells were pretreated with 20 μM HSF1A for 15 h, before the addition of 0.5 mM palmitate/BSA, or BSA alone, followed by a 15 h incubation. (D) Total protein was isolated from INS 832/13 cells treated as in (C) and analyzed as in (B). (E) NIH3T3 cells were pretreated with 50 μM HSF1A for 15 h, before the addition of 5 mM homocysteine followed by a 15 h incubation. (F) Total protein was isolated from NIH3T3 cells treated as in (B). Cell viability was analyzed with Cell Titer Glo. See also Figure S1.

Figure 2. HSF1A directly binds the TRiC complex

(A) HeLa cell extracts were incubated with 100 μM HSF1A-Biotin, proteins purified with neutravidin-agarose, resolved by SDS-PAGE and analyzed by immunoblotting for HSF1, protein chaperones and TRiC subunits. (I, input; B, Biotin; HB, HSF1A-Biotin) (B) Purified bovine TRiC and recombinant human Hsp70 were incubated with 100 μM HSF1A-Biotin, interacting proteins analyzed as in (A). (C) E. coli extracts expressing the indicated yeast TRiC subunits Tcp1, Cct2, Cct5 or Cct8 fused with FLAG tag were incubated with neutravidin-agarose beads only (B, bead) or with 100 μM HSF1A-Biotin (HB) and immunoblotted. (D) GST-Tcp1 was incubated with either DMSO (B), 100 μM HSF1A-Biotin (HB), or 10 μM Geldanamycin-Biotin (GB) and purified with neutravidin-agarose. As a control, GST-Tcp1 was also purified using glutathione-coated agarose beads (GSH). Purified protein was resolved by SDS-PAGE and analyzed by immunoblotting using Tcp1 antibody. (E) Purified bovine TRiC was incubated with DMSO or HSF1A-Biotin or Biotin and TRiC melting was analyzed by thermal denaturation profiling in the presence of SYPRO orange. (F) Fluorescence anisotropy was used to assess the affinity of HSF1A-FITC for purified recombinant Tcp1. Increased fluorescent polarization (mP) indicates binding to HSF1A-FITC in the presence of increasing concentrations of Tcp1 (G) His₆-Tcp1 (See also Figure S2C) was incubated with 0.5 μM Biotin or HSF1A-Biotin (HSF1A-B) and captured with neutravidin-agarose beads and immunoblotted.

Figure 3. HSF1A inhibits TRiC in vitro

(A) Purified TRiC was treated with DMSO or HSF1A and used in actin refolding assays. Folded actin was captured using a DNaseI-agarose resin, resolved by SDS-PAGE and analyzed and quantitated by autoradiography. (B) Relative ATP hydrolysis (normalized ratio ADP/ATP) by purified bovine TRiC in the presence of DMSO or 200 uM HSF1A over time (min).

Figure 4. HSF1A binding to Cct4 requires a hinge region

(A) Total cell lysate from HEK293T cells expressing human Cct4-GFP was incubated with ATP-sepharose, bound proteins eluted with ATP and eluate analyzed for Cct4 and GFP by immunobloting. (B) Total cell lysate from HEK293T cells expressing an empty vector, GFP, Cct4 or Cct4-GFP were incubated with ATP-sepharose and captured proteins eluted with ATP. GFP fluorescence was measured. (C) Total cell lysate from HEK293T cells expressing Cct4-GFP was incubated with ATP-sepharose as in (A) and washed with Biotin, HSF1A-Biotin or ATP. Eluted proteins were analyzed for Cct4-GFP by immunoblotting with anti-GFP antibody. (D) Protein elutions from (C) were measured for GFP fluorescence. (E) Tcp1 protein map (Kalisman et al., 2013) representing the three fragments D1 (1–152 aa), D2 (153–379 aa) and D3 (380–559 aa) used to analyze Tcp1-HSF1A interactions. (F) E. coli extract expressing yeast Tcp1 fused with a His₆ tag was incubated with neutravidin-agarose beads only (beads), 100 μM Biotin or HSF1A-Biotin (HSF1A-B) and immunoblotted with anti-His tag antibody. (G) E. coli extracts expressing yeast Tcp1 fragments D1, D2 or D3 fused to a His₆ tag were incubated with neutravidin-agarose beads only (beads), 100 μM Biotin or HSF1A-Biotin (HSF1A-B) and immunoblotted with anti-His tag antibody. (H) E. coli extracts expressing yeast Tcp1-D3 (380–559 aa) or the truncations Tcp1-D3-Tr1 (397–559 aa) and Tcp1-D3-Tr2 (404–559 aa) were incubated with 100 μM Biotin or HSF1A-Biotin (HSF1A-B), purified with neutravidin-agarose beads and immunoblotted with anti-His tag antibody. (I) E. coli extracts expressing the yeast Tcp1-D3 fragment with the triple point mutations LDE395:397AAA, DSL404:406AAA or GGG421:423AAA fused with a FLAG tag were incubated with 100 μM Biotin or HSF1A-Biotin (HSF1A-B), purified with neutravidin-agarose beads and immunoblotted with anti-FLAG antibody.

Figure 5. HSF1A inhibits TRiC function in vivo

(A) Wild-type, tcp1-DAmP and cct8-DAmP S. cerevisiae strains were treated with DMSO or 10 μM HSF1A, grown at 30 °C or 37 °C for 10 h and growth monitored by optical density. Data shown are growth rate of HSF1A treated cultures as a function of DMSO treated cultures. (B) NIH3T3 cells were treated with DMSO or HSF1A for 15 h or heat shocked for 2 h at 42 °C followed by a 15 h recovery at 37 °C. Total protein was analyzed for Hsp70, HSF1, Cct2, Cct3, α-tubulin, actin and VHL expression by immunoblotting. (C) HeLa cells transfected with a plasmid expressing HA-VHL were treated with DMSO or 80 μM HSF1A for 1 h. HA-VHL and interacting proteins were captured using an anti-HA agarose resin and analyzed by immunobloting for Hsp70, Cct3, Cct8 and VHL. (D) Immunoprecipitated (IP) protein and input (in) protein levels from (C) were quantified and IP protein levels normalized using input protein levels. Data are shown as a percent of IP protein levels of HSF1A treated cells versus DMSO treated cells.

Figure 6. TRiC inhibits human HSF1 activation in yeast and mammalian cells

(A) S. cerevisiae strain DNY248 expressing human HSF1 (hHSF1) or an empty vector (V) were transformed with plasmids expressing the indicated TRiC subunits and grown on SC-His or SC-His supplemented with 5-FOA. (B) S. cerevisiae strain DNY227 harboring an SSA3-β-galactosidase fusion gene was treated with 25 μM HSF1A or DMSO for 6 h or heat shocked at 39 °C for 3 h. Reporter gene activation was assessed by β-galactosidase activity assays. (C) Yeast strain BY4741 expressing a plasmid-borne SSA3 promoter-β-galactosidase fusion gene was transformed with either an empty vector or plasmids expressing the CCT5 or CCT8 genes. (D) Yeast-based assay scheme for impact of TRiC dysfunction on human HSF1 activation. Shown is the HSF1-LexA fusion protein bound to a LexA operator site upstream of the lacZ gene. See also Figure S4C. (E) Yeast strains BJ2168 (WT) and MA6 (CCT6 D89E) were transformed with a plasmid expressing HSF1-LexA and a LexA operator-dependent β-galactosidase reporter gene (left) or an SSA3-β-galactosidase fusion gene (right). Reporter gene activation was assessed by β-galactosidase activity assays. (F) HeLa cells treated for 72 h with siRNA against TCP1 or CCT3, or scrambled siRNA were analyzed for Hsp70, Tcp1, Cct3 and GAPDH levels by immunoblotting and (G) for Hsp70 levels (ng/mL) by ELISA. (H) NIH3T3 cells were transiently transfected with a plasmid expressing mouse TCP1 (CMV-TCP1) or vector control (VEC) and analyzed for Hsp70, Tcp1 and GAPDH protein levels by immunobloting. (I) NIH3T3 cells were treated as in (H) and Hsp70 transcript levels quantitated by qRT-PCR normalized to GAPDH.

Figure 7. TRiC directly interacts with HSF1

(A) HEK293T cells transfected with a plasmid expressing FLAG-HSF1 or an empty vector (Vec), were treated with DSP (+) or left untreated (−). HSF1 was immunoprecipitated from total protein extract using anti-FLAG-affinity resin and analyzed for HSF1, Hsp90, Hsp70, Cct2 and Cct3 by immunobloting. * indicates non-specific band. (B) HEK293T cells transfected with mouse HSF1-GFP-TAP or an empty vector were treated with DSP and HSF1 from total protein extract using anti-GFP-affinity resin. The immunoprecipitate was analyzed for HSF1, Hsp90, Hsp70, Cct3 by immunoblotting. (I, input; PD, pull-down) (C) Purified bovine TRiC, His₆-tagged HSF1ΔLZ1-3 or His₆-tagged wild-type HSF1 were incubated either alone or with the indicated combinations. Proteins were captured using cobalt-agarose resin and analyzed for HSF1, Cct2 and Cct3 by immunoblotting. Shown are images from the same blot that was separately probed with the indicated antibodies (D) In vitro TRiC – HSF1 interaction experiment carried out as in (C) but purified TRiC was pre-incubated with 200 μM HSF1A-Biotin or Biotin alone prior to addition of HSF1. (E) NIH3T3 and HEK293T cells transfected with a plasmid expressing FLAG-HSF1 were treated with 80 and 100 μM HSF1A, respectively, or DMSO for 6 h, cross-linked with DSP, HSF1 immunoprecipitated and immunoblotted as in (A) for HSF1, Hsp90, Hsp70 and Cct3. (F) Model for the repressive interaction between TRiC and HSF1 and its modulation by HSF1A.