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Abstract In response to various intracellular stress or damage-associated signals, inflammasomes can be activated and trigger a pyroptotic cell death process through the sequential assembly of structurally compatible and interacting filamentous oligomers involving the pyrin domains (PYD) of important inflammasome components. The PYD-containing interferon-inducible protein 16 (IFI16) has been suggested as a viral DNA sensor that can induce inflammasome formation, but it also has other inflammasome-independent functions, including interferon production. Here, the cryo-EM structure of the filament assembled by the PYD of human IFI16 reveals a helical architecture distinct from inflammasome PYD filaments. In silico interface energy calculations suggest that the helical architecture of the IFI16^PYDfilament prevents interactions with inflammasome PYD filaments. Biochemical and cell biology experiments consistently demonstrate that IFI16 does not directly interact with inflammasome pyrin domains. Together, our results provide insights into the structural basis of the inflammasome-independent functions of IFI16, and also show that strict architectural compatibility requirements for interactions contribute to the signal transduction specificity in inflammasome signaling. 

Cannulae are structurally rigid tubular protein filaments that accumulate on the extracellular surface of archaea within the family Pyrodictiaceae during cell growth. These obligate anaerobes propagate under hyperthermophilic conditions in which cannulae form a biomatrix that interconnects and sustains cells. The persistence of cannulae in this environment suggests that these filaments display significant thermostability, which has attracted technological interest in their development as synthetic protein-based biomaterials. Here, we report cryoEM structural analyses of ex vivo and in vitro assembled recombinant cannulae. We demonstrate that the interactions between protomers in native and recombinant cannulae is based on donor strand complementation (DSC), a form of non-covalent polymerization previously observed for bacterial chaperone-usher pili. Unexpectedly, calcium ion coordination at the subunit interfaces reinforces the network of donor strand interactions in the cannulae. This study provides insight into the mechanism of assembly of cannulae and the structural origin of their high stability and rigidity. 

Nucleoside triphosphate (NTP)-dependent protein assemblies such as microtubules and actin filaments have inspired the development of diverse chemically fueled molecular machines and active materials but their functional sophistication has yet to be matched by design. Given this challenge, we asked whether it is possible to transform a natural adenosine 5′-triphosphate (ATP)-dependent enzyme into a dissipative self-assembling system, thereby altering the structural and functional mode in which chemical energy is used. Here we report that FtsH (filamentous temperature-sensitive protease H), a hexameric ATPase involved in membrane protein degradation, can be readily engineered to form one-dimensional helical nanotubes. FtsH nanotubes require constant energy input to maintain their integrity and degrade over time with the concomitant hydrolysis of ATP, analogous to natural NTP-dependent cytoskeletal assemblies. Yet, in contrast to natural dissipative systems, ATP hydrolysis is catalyzed by free FtsH protomers and FtsH nanotubes serve to conserve ATP, leading to transient assemblies whose lifetimes can be tuned from days to minutes through the inclusion of external ATPases in solution.