Structural basis of nucleosome-dependent cGAS inhibition

Saving a host cell from itself A fundamental mammalian defense mechanism against pathogens and damaged cellular DNA is to recognize DNA fragments in the cytosol and trigger an inflammatory response. The cyclic guanosine monophosphate–adenosine monophosphate synthase (cGAS) that recognizes cytosolic DNA is also found in the nucleus, but here its activity is suppressed by tethering to chromatin. Two papers now report cryo–electron microscopy structures of cGAS bound to the nucleosome core particle (NCP). Kujirai et al. observed a structure with two cGAS molecules bridging two NCPs, whereas Boyer et al. observed cGAS bound to a single nucleosome. Together, these structures show how cGAS is prevented from autoreactivity toward host DNA. Science , this issue p. 455 , p. 450