Genome Protection by the 9-1-1 Complex Subunit HUS1 Requires Clamp Formation,DNA Contacts,and ATR Signaling-independent Effector Functions

The RAD9A-HUS1-RAD1 (9-1-1) complex is a heterotrimeric clamp that promotes checkpoint signaling and repair at DNA damage sites. In this study, we elucidated HUS1 functional residues that drive clamp assembly, DNA interactions, and downstream effector functions. First, we mapped a HUS1-RAD9A interface residue that was critical for 9-1-1 assembly and DNA loading. Next, we identified multiple positively charged residues in the inner ring of HUS1 that were crucial for genotoxin-induced 9-1-1 chromatin localization and ATR signaling. Finally, we found two hydrophobic pockets on the HUS1 outer surface that were important for cell survival after DNA damage. Interestingly, these pockets were not required for 9-1-1 chromatin localization or ATR-mediated CHK1 activation but were necessary for interactions between HUS1 and its binding partner MYH, suggesting that they serve as interaction domains for the recruitment and coordination of downstream effectors at damage sites. Together, these results indicate that, once properly loaded onto damaged DNA, the 9-1-1 complex executes multiple, separable functions that promote genome maintenance.     

Protein toxins from plants and bacteria: Probes for intracellular transport and tools in medicine

A number of protein toxins produced by bacteria and plants enter eukaryotic cells and inhibit protein synthesis enzymatically. These toxins include the plant toxin ricin and the bacterial toxin Shiga toxin, which we will focus on in this article. Although a threat to human health, toxins are valuable tools to discover and characterize cellular processes such as endocytosis and intracellular transport. Bacterial infections associated with toxin production are a problem worldwide. Increased knowledge about toxins is important to prevent and treat these diseases in an optimal way. Interestingly, toxins can be used for diagnosis and treatment of cancer.     

In vitro assessment of a chemically synthesized Shiga toxin receptor analog attached to chromosorb P (Synsorb Pk) as a specific absorbing agent of Shiga toxin 1 and 2.

A synthetic analog of Shiga toxin (Stx) receptor (Synsorb Pk) was quantitatively assessed to determine whether it can protect human renal adenocarcinoma cells (ACHN cells) from the cytotoxicity of Stx1 and Stx2 by coincubation experiments. Coincubation of 100 and 20 ng of Stxl and Stx2 with 50 mg of Synsorb Pk for 1 hr at 37 C in 1 ml of Eagle's Minimum Essential Medium supplemented with 1% (v/v) non-essential amino acid and 10% (v/v) fetal calf serum protected 50% of the cells from the cytotoxic effect. Chromosorb P, an inert matrix control, did not absorb the Stxs at all. Heat-treatment (boiled for 10 min) to Synsorb Pk caused a 50% decrease in Stx2-binding activity, but did not effect the Stx1 binding. Further, Stxs bound to Synsorb Pk could be demonstrated. When 20 mg of Synsorb Pk was coincubated for 30 min at 37 C in 1 ml of phosphate-buffered saline with 1 and 10 ng or more of Stx1 or Stx2, respectively, the toxins could be detected on the surface when the bound toxins on Synsorb Pk were used as the solid phase in enzyme immunoassay. The amount of 100 ng/ml of both Stxl and Stx2 appeared to saturate 20 mg/ml of Synsorb Pk after coincubating for 30 min at 37 C. While assessing the Stxs' binding activity to Synsorb Pk, it was demonstrated that Stxl had a higher affinity to Pk trisaccharide than Stx2. These observations provide useful information on the effectiveness of Synsorb Pk to trap and eliminate free Stxs produced in the gut of patients infected by Stx-producing Escherichia coli, and to prevent the progression of hemorrhagic colitis to hemolytic uremic syndrome.