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Biochemical and structural basis of toxicity and immunity in contact-dependent growth inhibition (CDI) systems
D. A. LOW (1), Z. Ruhe (1), C. Hayes (1), S. Koskiniemi (2), C. Goulding (3), S. Poole (1). (1) University of California, Santa Barbara, CA, U.S.A.; (2) Uppsala University, Uppsala, Sweden; (3) University of California, Irvine, CA, U.S.A.

Many bacterial pathogens, including some that attack plants, express one or more systems that block the growth of neighboring cells that are recognized as "non-self". Systems designated as "CDI" (contact-dependent growth inhibition) inhibit the growth of neighboring cells via expression of large extended beta-helical structures, called CdiA, on their surfaces. CdiA proteins recognize specific receptors on susceptible target bacteria. CdiA(EC93) from E. coli EC93 uses the highly conserved outer-membrane protein, BamA, as its receptor, but other CdiA proteins bind different receptor molecules. Upon binding its receptor, CdiA delivers a C-terminal toxin domain (CdiA-CT) into the target bacterium. Many CDI toxins are nucleases, and thus must traverse both the outer- and inner membrane to gain access to their target substrates. CDI+ cells protect themselves from toxin activity by producing a CdiI immunity protein encoded immediately downstream of cdiA. CdiI binds to the CdiA-CT toxin and neutralizes its activity. Each CdiI immunity protein binds specifically to its cognate CdiA toxin, enabling self-recognition. Further recognition occurs at the receptor binding level. Polymorphic extracellular loops of BamA serve as binding sites for CdiA(EC93). This restricts this E. coli CDI system to delivering toxin only to other E. coli cells that express BamA with appropriate loop sequences.

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