Death domain fold proteins in immune signaling and transcriptional regulation

Death domain fold (DDF) superfamily comprises of the death domain (DD), death effector domain (DED), caspase activation recruitment domain (CARD), and pyrin domain (PYD). By utilizing a conserved mode of interaction involving six distinct surfaces, a DDF serves as a building block that can densely pack into homomultimers or filaments. Studies of immune signaling components have revealed that DDF-mediated filament formation plays a central role in mediating signal transduction and amplification. The unique ability of DDFs to self-oligomerize upon external signals and induce oligomerization of partner molecules underlies key processes in many innate immune signaling pathways, as exemplified by RIG-I-like receptor signalosome and inflammasome assembly. Recent studies showed that DDFs are not only limited to immune signaling pathways, but also are involved with transcriptional regulation and other biological processes. Considering that DDF annotation still remains a challenge, the current list of DDFs and their functions may represent just the tip of the iceberg within the full spectrum of DDF biology. In this review, we discuss recent advances in our understanding of DDF functions, structures, and assembly architectures with a focus on CARD- and PYD-containing proteins. We also discuss areas of future research and the potential relationship of DDFs with biomolecular condensates formed by liquid-liquid phase separation (LLPS).

The PYRIN domain in signal transduction

The Death Domain Fold superfamily of evolutionarily conserved protein-protein interaction domains consists of 4 subfamilies: the death domain, the death effector domain, the caspase recruitment domain, and the PYRIN domain. Interaction of Death Domain Fold containing proteins modulates the activity of several downstream effectors, such as caspases and transcription factors. Recent studies provide evidence for not only homotypic-, but also heterotypic interactions among different sub-families, and even unconventional non-death domain fold interactions. As the number of potential protein associations among Death Domain Fold containing proteins expands and their influence on cellular responses increases, a challenging field for new investigations opens up. This review will focus on PYRIN domain-containing proteins and discuss the recent advances that provide strong evidence that PYRIN domain-mediated signal transduction has broad implications on cellular functions, including innate immunity, inflammation, differentiation, apoptosis, and cancer.