Structure and Function of MARTX Toxins and Other Large Repetitive RTX Proteins

The Repeats-in-Toxins (RTX) family of proteins classically consists of cytolysins and hemolysins. Over the past decade, genome sequencing revealed the existence of very large members of this family. These are all repetitive proteins ranging in size from 200 to 900 kDa that function as toxins or adhesins. Many are exported by Type I secretion. One major new subfamily is the large repetitive RTX adhesins and biofilm-associated proteins. These are characterized by 80- to 300-amino-acid repeats ordered in tandem, although the sequence and number of the repeats vary by protein. The second major new subfamily is the multifunctional-autoprocessing RTX toxins, which are associated with cytotoxicity and pathogenesis. These proteins are in turn distantly related to Yersinia hypothetical RTX proteins that may autoprocess by a similar mechanism. This review discusses current knowledge regarding the structure and function of these new subfamilies of RTX proteins.