C-terminal (CT) domain of the contact-dependent growth inhibition (CDI) system protein CdiA (CdiA-CT) of Yersinia kristensenii, 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 C-terminal (CT) toxin domain of CdiA effector protein from Yersinia kristensenii 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 the C-terminal domain. A wide variety of C-terminal toxin domains appear to exist; Yersinia kristensenii CdiA-CT has potent RNase activity in vivo and in vitro. Although CdiA-CT has structural homology with angiogenin and other RNase A paralogs, it does not share sequence similarity with these nucleases and lacks the characteristic disulfide bonds of the superfamily. It binds its cognate immunity protein CdiI which neutralizes toxicity by blocking access to RNA substrates. Y. kristensenii CdiA-CT is the first non-vertebrate protein found to possess the RNase A superfamily fold. Homologs of this toxin are associated with secretion systems in many Gram-negative and Gram-positive bacteria, suggesting that RNase A-like toxins are commonly deployed in inter-bacterial competition.