ce angle of the reflected light which is recorded as the refractive index. An increase in the refractive index unit was detected when PBA was perfused over the CT and CTA1 sensor slides. However, no change in the RIU was recorded when 100 mM PBA was perfused over the CTB5 sensor slide. A strong positive signal was obtained when an anti-CTB antibody 2 April 2011 | Volume 6 | Issue 4 | e18825 Use of PBA as a Toxin Inhibitor Binding Parameter PBA+CT PBA+CTA1 ka 1.21610 5 kd 1.38610 23 KD 11 9 1.556105 1.3161023 doi:10.1371/journal.pone.0018825.t001 tryptophan fluorescence with a Tm of 36uC for CTA1 and a Tm of 41uC for PBA-treated CTA1. As assessed by far-UV CD, the toxin secondary structure exhibited a Tm of 35uC for reduced CTA1/CTA2 and a Tm of 44uC for reduced and PBA-treated CTA1/CTA2. These results demonstrated that PBA inhibits the thermal perturbation of both CTA1 secondary and tertiary structures. CD and fluorescence spectroscopy measurements were also conducted with reduced CTA1/CTA2 heterodimers incubated in the presence of 1 or 10 mM PBA. The Tm values derived from all of our biophysical experiments are presented in 3 April 2011 | Volume 6 | Issue 4 | e18825 Use of PBA as a Toxin Inhibitor Transition Temperature mM PBA Near-UV CD 30.0 33.0 35.0 36.0 Fluorescence Spectroscopy 36.0 37.5 38.5 41.0 Far-UV CD 35.0 37.0 39.0 44.0 thermally disordered CTA1 subunit. No significant changes to the structure of CTA1 occurred five minutes or one hour after the addition of PBA at 37uC. Cooling CTA1 from 37uC to 18uC will allow the toxin to regain a folded conformation, so the unfolding of CTA1 at 37uC is a reversible process. Thus, PBA will stabilize the folded CTA1 subunit but will not facilitate the renaturation of an unfolded CTA1 subunit. 0 10516638 1 10 100 PBA inhibits CTA1 translocation to the cytosol Our collective data suggested that PBA can bind to holotoxinassociated CTA1 and can then prevent the spontaneous thermal unfolding of the dissociated CTA1 subunit. By our model, CTA1 stabilization would prevent its recognition by the ERAD system and its ERAD-mediated translocation to the cytosol. To test this prediction, we performed a translocation assay to monitor the ERto-cytosol export of CTA1 in the absence or presence of PBA. Surface-bound CT was chased into HeLa cells for two hours WP 1130 before organelle and cytosolic fractions were generated from the intoxicated cells. Western blot controls demonstrated that protein disulfide isomerase, a soluble ER resident protein, was found only in the pellet fraction which contained intact membrane-bound organelles. As expected, the cytosolic protein Hsp90 was found in the supernatant fraction which contained the cytosol. Our protocol could thus effectively segregate the cell extracts into distinct organelle and cytosolic fractions. Only a minor pool of surface-bound CT is transported to the ER; the majority of internalized toxin is instead degraded in the lysosomes. We accordingly used the highly sensitive method of SPR to detect CTA1 in the cytosolic fractions from untreated and PBA-treated cells. For this experiment, SPR sensor slides were coated with an anti-CTA antibody. The cytosolic fractions from our cell extracts were then perfused over a sensor slide in order to detect the translocated, cytosolic pool of CTA1. No signal was obtained from the cytosol of unintoxicated cells or from the cytosol of cells intoxicated in the presence of brefeldin A, a drug that blocks toxin transport to the ER translocce angle of the reflected light which is recorded as the refractive index. An increase in the refractive index unit was detected when PBA was perfused over the CT and CTA1 sensor slides. However, no change in the RIU was recorded when 100 mM PBA was perfused over the CTB5 sensor slide. A strong positive signal was obtained when an anti-CTB antibody 2 April 2011 | Volume 6 | Issue 4 | e18825 Use of PBA as a Toxin Inhibitor Binding Parameter PBA+CT PBA+CTA1 ka 1.21610 5 kd 
1.38610 23 KD 11 9 1.556105 1.3161023 doi:10.1371/journal.pone.0018825.t001 tryptophan fluorescence with a Tm of 36uC for CTA1 and a Tm of 41uC for PBA-treated CTA1. As assessed by far-UV CD, the toxin secondary structure exhibited a Tm of 35uC for reduced CTA1/CTA2 and a Tm of 44uC for reduced and PBA-treated CTA1/CTA2. These results demonstrated that PBA inhibits the thermal perturbation of both CTA1 secondary and tertiary structures. CD and fluorescence spectroscopy measurements were also conducted with reduced CTA1/CTA2 heterodimers incubated in the presence of 1 or 10 mM PBA. The Tm values derived from all of our biophysical experiments are presented in 3 April 2011 | Volume 6 | Issue 4 | e18825 Use of PBA as a Toxin Inhibitor Transition Temperature mM PBA Near-UV CD 30.0 33.0 35.0 36.0 Fluorescence Spectroscopy 36.0 37.5 38.5 41.0 Far-UV CD 35.0 37.0 39.0 44.0 thermally disordered CTA1 subunit. No significant changes to the structure of CTA1 occurred five minutes or one hour after the addition of PBA at 37uC. Cooling CTA1 from 37uC to 18uC will allow the toxin to regain a folded conformation, so the unfolding of CTA1 at 37uC is a reversible process. Thus, PBA will stabilize the folded CTA1 subunit but will not facilitate the renaturation of an unfolded CTA1 subunit. 0 1 10 100 PBA inhibits CTA1 translocation to the cytosol Our collective data suggested that PBA can bind to holotoxinassociated CTA1 and can then prevent the spontaneous thermal unfolding of the dissociated CTA1 subunit. By our model, CTA1 stabilization would prevent its recognition by the ERAD system and its ERAD-mediated translocation to the cytosol. To test this prediction, we performed a translocation assay to monitor the ERto-cytosol export of CTA1 in the absence or presence of PBA. Surface-bound CT was chased into HeLa cells for two hours before organelle and cytosolic fractions were generated from 17942897 the intoxicated cells. Western blot controls demonstrated that protein disulfide isomerase, a soluble ER resident protein, was found only in the pellet fraction which contained intact membrane-bound organelles. As expected, the cytosolic protein Hsp90 was found in the supernatant fraction which contained the cytosol. Our protocol could thus effectively segregate the cell extracts into distinct organelle and cytosolic fractions. Only a minor pool of surface-bound CT is transported to the ER; the majority of internalized toxin is instead degraded in the lysosomes. We accordingly used the highly sensitive method of SPR to detect CTA1 in the cytosolic fractions from untreated and PBA-treated cells. For this experiment, SPR sensor slides were coated with an anti-CTA antibody. The cytosolic fractions from our cell extracts were then perfused over a sensor slide in order to detect the translocated, cytosolic pool of CTA1. No signal was obtained from the cytosol of unintoxicated cells or from the cytosol of cells intoxicated in the presence of brefeldin A, a drug that blocks toxin transport to the ER transloc
