The nucleotidyl cyclase toxin ExoY is one of the virulence factors injected by the Pseudomonas aeruginosa type III secretion system into host cells. Inside cells, it is activated by an unknown eukaryotic cofactor to synthesize various cyclic nucleotide monophosphates. ExoY-like adenylate cyclases are also found in Multifunctional-Autoprocessing Repeats-in-ToXin (MARTX) toxins produced by various Gram-negative pathogens. Here we demonstrate that filamentous actin (F-actin) is the hitherto unknown cofactor of ExoY. Association with F-actin stimulates ExoY activity more than 10,000 fold in vitro and results in stabilization of actin filaments. ExoY is recruited to actin filaments in transfected cells and alters F-actin turnover. Actin also activates an ExoY-like adenylate cyclase MARTX effector domain from Vibrio nigripulchritudo. Finally, using a yeast genetic screen, we identify actin mutants that no longer activate ExoY. Our results thus reveal a new sub-group within the class II adenylyl cyclase family, namely actin-activated nucleotidyl cyclase (AA-NC) toxins.

Bacterial nucleotidyl cyclase toxins are potent virulence factors that upon entry into eukaryotic cells are stimulated by endogenous cofactors to catalyse the production of large amounts of 3'5'-cyclic nucleoside monophosphates. The activity of the effector ExoY from Pseudomonas aeruginosa is stimulated by the filamentous form of actin (F-actin). Utilising yeast phenotype analysis, site-directed mutagenesis, functional biochemical assays and confocal microscopy, we demonstrate that the last 9 amino acids of the C-terminus of ExoY are crucial for the interaction with F-actin and, consequently, for ExoY’s enzymatic activity in vitro and toxicity in a yeast model. We observed that isolated C-terminal sequences of P. aeruginosa ExoY, that had been fused to a carrier protein, bind to F-actin, and that synthetic peptides corresponding to the extreme ExoY C-terminus inhibit ExoY enzymatic activity in vitro and compete with the full-length enzyme for F-actin binding. Interestingly, we noted that various P. aeruginosa isolates of the PA14 family, including highly virulent strains, harbour ExoY variants with a mutation altering the C-terminus of this effector. We found that these naturally occurring ExoY variants display drastically reduced enzymatic activity and toxicity. Our findings shed light on the molecular basis of the ExoY–F-actin interaction, revealing that the extreme C terminus of ExoY is critical for binding to F-actin in target cells and that some P. aeruginosa isolates carry C-terminally mutated, low-activity ExoY variants.

Many pathogens manipulate host cell cAMP signaling pathways to promote their survival and proliferation. Bacterial Exoenzyme Y (ExoY) toxins belong to a family of invasive, structurally-related bacterial nucleotidyl cyclases (NC). Inactive in bacteria, they use proteins that are uniquely and abundantly present in eukaryotic cells to become potent, unregulated NC enzymes in host cells. Other well-known members of the family include Bacillus anthracis Edema Factor (EF) and Bordetella pertussis CyaA. Once bound to their eukaryotic protein cofactor, they can catalyze supra-physiological levels of various cyclic nucleotide monophosphates in infected cells. Originally identified in Pseudomonas aeruginosa, ExoY-related NC toxins appear now to be more widely distributed among various γ- and β-proteobacteria. ExoY-like toxins represent atypical, poorly characterized members within the NC toxin family. While the NC catalytic domains of EF and CyaA toxins use both calmodulin as cofactor, their counterparts in ExoY-like members from pathogens of the genus Pseudomonas or Vibrio use actin as a potent cofactor, in either its monomeric or polymerized form. This is an original subversion of actin for cytoskeleton-targeting toxins. Here, we review recent advances on the different members of the NC toxin family to highlight their common and distinct functional characteristics at the molecular, cytotoxic and enzymatic levels, and important aspects that need further characterizations.

The Type III Secretion System (T3SS) of Pseudomonas aeruginosa injects effector proteins into host cells to subvert cellular processes and promote infection. While the roles of several T3SS effectors in virulence are well established, the function of the nucleotidyl cyclase ExoY has remained elusive and debated. Here, we show that ExoY-produced cyclic GMP (cGMP) regulates the cytotoxic activity of the co-injected effector ExoT, thereby modulating host cell damage. Using engineered P. aeruginosa strains expressing ExoY variants with distinct substrate preferences, we demonstrate that cGMP production by ExoY limits ExoT-mediated dephosphorylation of the host adaptor protein CrkII, a downstream consequence of ExoT’s ADP-ribosyltransferase activity. This attenuation reduces ExoT-induced cell retraction and decreases bacterial virulence. In contrast, we find that ExoT and another T3SS effector, ExoS, can inhibit ExoY activity and cGMP production in certain cell types, thus limiting ExoY’s regulatory influence. These findings highlight the intricate interplay between T3SS effectors within host cells and reveal an unrecognized layer of ExoY-based complexity in P. aeruginosa ’s pathogenic strategy.