Thermodynamics and stability of the PAAD/DAPIN/PYRIN domain of IFI-16.

The PAAD domain is a conserved domain recently identified in more than 35 human proteins that are involved in apoptosis and inflammatory signaling pathways. Structural studies have confirmed that this domain belongs to the death domain superfamily which includes PAAD/CARD/DED/DD families. Recently, the 3D structures determined by NMR of NALP1 and ASC PAAD domain, members of the PAAD family, have shown that it is composed of a 6 helix bundle as with other death domain family members. However, helix-3 in the solved structures is unordered in solution. In this study we compare the thermodynamic, folding and stability properties of different members of the PAAD and CARD families and investigate structural conformational changes induced by the helix inducers trifluoroethanol and SDS on the PAAD domain of IFI16 and on the CARD domain of RAIDD. We show that inside the PAAD and CARD families, members have similar thermodynamic properties, however, the DeltaG of folding for PAAD and CARD members are, respectively, -1.4 and -5.5 kcal mol(-1). This difference is attributed to less alpha helical content for PAAD due to the unfolding of helix-3 that lowers bonded energy and increases disorder when compared to CARD members. Despite identical fold between PAAD and CARD families but limited sequence identity, there are striking differences in the thermodynamics of both families.