A comprehensive interaction study provides a potential domain interaction network of human death domain superfamily proteins

Human death domain superfamily proteins (DDSPs) play important roles in many signaling pathways involved in cell death and inflammation. Disruption or constitutive activation of these DDSP interactions due to inherited gene mutations is closely related to immunodeficiency and/or autoinflammatory diseases; however, responsible gene mutations have not been found in phenotypical diagnosis of these diseases. In this study, we comprehensively investigated the interactions of death-fold domains to explore the signaling network mediated by human DDSPs. We obtained 116 domains of DDSPs and conducted a domain–domain interaction assay of 13,924 reactions in duplicate using amplified luminescent proximity homogeneous assay. The data were mostly consistent with previously reported interactions. We also found new possible interactions, including an interaction between the caspase recruitment domain (CARD) of CARD10 and the tandem CARD–CARD domain of NOD2, which was confirmed by reciprocal co-immunoprecipitation. This study enables prediction of the interaction network of human DDSPs, sheds light on pathogenic mechanisms, and will facilitate identification of drug targets for treatment of immunodeficiency and autoinflammatory diseases.

Structure-based alignment of human caspase recruitment domains provides a framework for understanding their function

Intracellular signalling is driven by protein-protein interactions. Members of the Death Domain superfamily mediate protein-protein interactions in both cell death and innate immune signalling pathways. They drive the formation of macromolecular complexes that act as a scaffold for protein recruitment and downstream signal transduction. Death Domain family members have low sequence identity, complicating their identification and predictions of their structure and function. We have taken all known human caspase recruitment domains (CARDs), a subfamily of the Death Domain superfamily, and generated a structure-guided sequence alignment. This alignment has enabled the identification of 14 positions that define the hydrophobic core and present a template for the identification of novel CARD sequences. We identify a conserved salt bridge in over half of all human CARDs and find a subset of CARDs likely to be regulated by tyrosine phosphorylation in their type I interface. Our alignment highlights that the CARDs of NLRC3 and NLRC5 are likely to be pseudodomains that have lost some of their original functionality. Together these studies demonstrate the benefits of structure-guided sequence alignments in understanding protein functionality.