RTX-toxins in Actinobacillus pleuropneumoniae and their potential role in virulence

Abstract

Actinobacillus pleuropneumoniae, the causative agent of swine pleuropneumonia, secretes hemolysins which are considered to play an important role in virulence. They are proteins with apparent molecular weights of 105 kDa. They are strongly immunogenic in naturally or experimentally infected pigs. The strongly hemolytic and cytolytic hemolysin I (HlyI) is produced by serotypes or strains which are particularly virulent, while the less hemolytic and cytolytic hemolysin II (HlyII) is produced by all serotypes except type 10. Several serotypes secrete both HlyI and HlyII which have the same apparent molecular weight, but which can be distinguished by monoclonal and polyclonal antibodies. The hemolysin I operon consists of a structural hlyl A gene encoding the prohemolysin, an activator gene hlyIC necessary for the activation of prohemolysin to active hemolysin and two genes hlyIB and hlyID encoding proteins for specific hemolysin secretion. This is a common feature of the group of RTX-toxins (repeats in the structural toxin) which are widely spread among human and animal pathogenic Gram-negative bacteria. The amino acid sequence of HlyI, which resembles the E. coli α-hemolysin, shows a membrane-active amphipathic helix at its N-terminus, followed by three strongly hydrophobic domains forming a typical transmembrane structure, and a segment of 13 repeated glycine-rich nonapeptides at the C-terminus of the protein which is known from other related RTX toxins to bind Ca2+. The hlyII operon only contains the hlyIIC and hlyII A genes. The proteins involved in the secretion of HlyII are provided by the hlyI operon. In those serotypes which produce HlyII but not HlyI, only part of the hlyI operon, containing the promoter, the hlyIB and hlyID secretion genes and a truncated hlyI A gene is found. The amino acid sequence of HlyII is more similar to the Pasteurella haemolytica leukotoxin than to HlyI and E. coli α-hemolysin. In contrast to HlyI, it only contains 8 glycine-rich nonapeptide repeats suggesting a lower Ca2+-binding capacity for HlyII than for HlyI. So far A. pleuropneumoniae is the first bacterium known to contain and express two different RTX-hemolysins. From structural data we assume that the two hemolysins recognize different target cells and have different tasks in bacteria-host interactions during infection. A third RTX-toxin named pleurotoxin with an apparent molecular weight of 120 kDa has no hemolytic, but strong cytotoxic activity and has been detected in some serotypes which are devoid of HlyI. The following new and uniform designations for A. pleuropneumoniae RTX-toxins are proposed: ApxI for HlyI (ClyI); Apx II for HlyII (ClyII, App); and ApxIII for Ptx (ClyIII).KeywordsHemolytic ActivityApparent Molecular WeightYersinia PestisStrong Cytotoxic ActivityPorcine Alveolar MacrophageThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.