inhibitor (or immunity protein) of the contact-dependent growth inhibition (CDI) system of Escherichia coli 536, and similar proteins
CDI toxins are expressed by gram-negative bacteria as part of a mechanism to inhibit the growth of neighboring cells. This model represents the inhibitor (CdiI, also called CdiI immunity protein) of the CdiA effector protein from Escherichia coli 536 (which is a predicted RNase), and similar proteins. CdiA secretion is dependent on the outer membrane protein CdiB. Upon binding to a receptor on the surface of target bacteria, the CDI toxin is delivered via its C-terminal domain (CdiA-CT). The CdiI immunity proteins are intracellular proteins that inactivate the toxin/effector protein to prevent auto-inhibition. They are specific for their cognate CdiA-CT and do not protect cells from the toxins of other CDI+ bacteria. Thus, CDI systems encode a complex network of toxin-immunity protein pairs that are deployed for intercellular competition. This E. coli CdiI's cognate toxin CdiA-CT domain is activated only when it is bound to the biosynthetic enzyme O-acetylserine sulfhydrylase-A (CysK), one of two isoenzymes (along with CysM) that catalyze the final reaction in cysteine synthesis. CdiA's predicted nuclease active site is occluded by immunity protein in the CysK/CdiA-CT/CdiI structure, suggesting that CdiI blocks the binding of tRNA substrates to the toxin.
Jiyao W et al.(2023). "The conserved domain database in 2023.",