Membrane Activity and Channel Formation of the Adenylate Cyclase Toxin (CyaA) of Bordetella pertussis in Lipid Bilayer Membranes

Abstract

The Gram-negative bacterium Bordetella pertussis is the cause of whooping cough. One of its pathogenicity factors is the adenylate cyclase toxin (CyaA) secreted by a Type I export system. The 1706 amino acid long CyaA (177 kDa) belongs to the continuously increasing family of repeat in toxin (RTX) toxins because it contains in its C-terminal half a high number of nine-residue tandem repeats. The protein exhibits cytotoxic and hemolytic activities that target primarily myeloid phagocytic cells expressing the αMβ2 integrin receptor (CD11b/CD18). CyaA represents an exception among RTX cytolysins because the first 400 amino acids from its N-terminal end possess a calmodulin-activated adenylate cyclase (AC) activity. The entry of the AC into target cells is not dependent on the receptor-mediated endocytosis pathway and penetrates directly across the cytoplasmic membrane of a variety of epithelial and immune effector cells. The hemolytic activity of CyaA is rather low, which may have to do with its rather low induced permeability change of target cells and its low conductance in lipid bilayer membranes. CyaA forms highly cation-selective channels in lipid bilayers that show a strong dependence on aqueous pH. The pore-forming activity of CyaA but not its single channel conductance is highly dependent on Ca2+ concentration with a half saturation constant of about 2 to 4 mM.

RTX-Toxins

“…LtxA (trade name Leukothera ® ) may provide therapeutic applications for the treatment of hematological malignancies and immune-mediated diseases [18]. All of the mentioned cytolysins form cation-selective pores in the cell membranes as the primary event [16][17][18]20,21,24,25]. This means that antibodies directed against specific RTX toxins could protect from diseases [21].…”

“…CyaA acts on target cells by uncontrolled production of cyclic adenosine monophosphate (cAMP) in the presence of calmodulin [7][8][9]17,19]. Similarly, the edema toxin (ETX) of Bacillus anthracis also produces calmodulin-dependent cAMP, which means that both toxins lead to a massive increase of this second messenger [19].…”

“…RTX proteins can be very large and may comprise more than 6000 amino acids [ 15 ]. RTX toxins are much smaller and contain between 1000 and 2000 amino acids [ 1 , 2 , 3 , 14 , 16 , 17 ]. Some RTX toxins, like the HlyA toxin of E. coli , act on a wide range of different host cell types, but others, e.g., the leukotoxins of A. actinomycetemcomitans , act only on a restricted number of cells in a species-specific fashion [ 18 ].…”

“…This Special Issue focuses on the function of RTX toxins of different subfamilies and their role in disease [ 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. Of special interest is the structure of the nonapeptide repeats and their interaction with calcium ions, which represents some prerequisites for target cell recognition [ 13 ].…”

“…In low calcium, cytoplasmic fluid, the repeat domains are unfolded and become folded in high calcium external fluid, which means that the repeats could act as chaperones [ 13 ]. On the other hand, it is clear that calcium-mediated conformation changes of the repeats play an important role in target cell recognition and in membrane-activity of certain RTX toxins [ 13 , 17 ]. This is not only the case for pore formation of cytolytic RTX toxins but also for the AMS8 lipase from the Antarctic Pseudomonas fluorescens strain AMS8 [ 23 ].…”