Protein: DP02878 (UniProt Q8XAD5)
Region: residues 1	to 15
Annotation: disorder (Structural state)
Evidence: nuclear magnetic resonance spectroscopy evidence used in manual assertion
Supporting statements:
Here, we present a combination of SAXS and NMR data to describe the conformational ensemble of the PaaA2 antitoxin from the human pathogen E. coli O157
We show that the conformational ensemble of PaaA2 is highly compact and that the protein exists in solution as two preformed helices, connected by a flexible linker, that probably act as molecular recognition elements for toxin inhibition
The fully assigned 1H-15N HSQC spectrum of PaaA2 shows that most of the backbone amide resonances are confined to a narrow spectral region (Figure 4A), typical for IDPs or proteins with a high α-helical content 
Chemical shift-based secondary structure predictions with DANGLE and TALOS+ suggest the presence of two a helices (residues Glu16 to Lys28 and His42 to Arg57), connected by a linker segment (Figure 4B), which is in agreement with our bioinformatics analysis.
On the other hand, δ2D (Camilloni et al., 2012), which is benchmarked for IDPs, identifies two stable cores existing in an α-helical conformation for more than 80% of the time: residues Asn22 to Arg26 and Glu46 to Glu51 for helices 1 and 2, respectively (Figure 4C). These core regions coincide with the predicted α helices and contain flanking regions with percentages of α helicity between 15% and 80% (Figure 4C), which is overall in agreement with the CD data (Table S1). All of the other residues are predicted to adopt a coil-like conformation. In agreement with these predictions, the NOE spectroscopy (NOESY) spectra indicate the absence of β sheet signatures and contain peaks characteristic of α helices.
Altogether, these findings reveal that, in the absence of its cognate binding partner ParE2, the PaaA2 antitoxin in solution harbors two α helices connected by a flexible linker
Although PaaA2 may assume a certain level of compactness, it is clear that the protein neither adopts a globular conformation nor contains a hydrophobic core. Although the former is supported by the analytical SEC and DLS experiments, the latter is evidenced by the thermal unfolding data.
Second, the structural study of PaaA2 reveals that the antitoxin is a monomer composed of two α helices connected by a highly flexible linker. Interestingly, these pre-existing α helices align well with the toxin-binding elements of the archetypal RelB antitoxins. Therefore, we hypothesize that these helices act as molecular recognition elements that allow PaaA2 to efficiently bind the ParE2 toxin. Given that the helices are preformed, PaaA2 probably interacts with its cognate toxin through the mechanism of conformational selection
The entire protein is an IDP, but regions 16-28 and 42-57 are in a helical conformation, constituting two preformed helices. The boundaries of these helices are not strictly defined as the core region is helical >80% of the time and the flanking regions have decreasing fraction of helicity as the helical content is highly dynamic in the ensemble. The boundary established by the authors in the text is taken, which is highly coincident with the models deposited in the PDB where the helices range from residues 14-29 and 43-57
Cross-references: PDB:3ZBE