RTX Toxins

Dear Colleagues,

RTX toxins (Repeats in ToXin) represent a steadily increasing family of gram-negative bacterial proteins containing functionally important glycine-rich and aspartate-containing nonapeptide repeats of the consensus sequence G-G-X-G-(N/D)-D-X-(L/I/F)-X (where X can be any amino acid). The repeats are able to bind calcium ions with high affinity to form a special structure that is important for target cell recognition. RTX proteins comprise many different functional categories. The most well-known of these is the RTX cytolysin family, where HlyA of uropathogenic Escherichia coli may represent the classical example of a pore-forming RTX toxin. Besides the cytolysins, the RTX proteins also contain subfamilies of many diverse functions. These subfamilies consist of RTX adhesins; enzymatic toxins; bacteriocins; surface layer proteins; and hydrolytic enzymes, such as proteases and lipases. More than 1000 RTX proteins are known to date. Common to all RTX proteins is their secretion via a type-one secretion system (T1SS) across the bacterial inner and outer membranes. This system is composed of different proteins that are encoded on the rtx-operon in transcriptional order. It comprises an inner membrane ATPase, a linker protein to combine inner and outer membrane components and an outer membrane pore. The rtx-operon may also code for an acyltransferase that modifies together with a bacterial acyl carrier protein the RTX toxins. The secreted RTX proteins have approximately a 60 amino acid-long secretion signal at the C-terminus.

This Special Issue will focus on the function of RTX toxins of different subfamilies. This comprises the cytolysin subfamily, which represents severe pathogenicity factors for diseases in humans or animals and other key virulence factor such as the adenylate cyclase toxin (Act, CyaA) of the whooping cough agent Bordetella pertussis. The large RTX protein family of the adhesins will also be a topic of this Special Issue together with biofilm associated RTX proteins.

Prof. Roland Benz
Guest Editor

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