Residues involved in the pore-forming activity of theClostridium perfringensiota toxin

Abstract

SummaryClostridium perfringens iota toxin is a binary toxin that is organized into enzyme (Ia) and binding (Ib) components. Ib forms channels in lipid bilayers and mediates the transport of Ia into the target cells. Here we show that Ib residues 334-359 contain a conserved pattern of alternating hydrophobic and hydrophilic residues forming two amphipathic β-strands involved in membrane insertion and channel formation. This stretch of amino acids shows remarkable structural and functional analogies with the β-pore-forming domain of C. perfringens epsilon toxin. Several mutations within the two amphipathic β-strands affected pore formation, single-channel conductance and ion selectivity (S339E-S341E, Q345H N346E) confirming their involvement in channel formation. F454 of Ib corresponds to the Φ-clamp F427 of anthrax protective antigen and F428 of C2II binary toxins. The mutation F454A resulted in a loss of cytotoxicity and strong increase in singlechannel conductance (500 pS as compared with 85 pS in 1 M KCl) with a slight decrease in cation selectivity, indicating that the Φ-clamp is highly conserved and crucial for binary toxin activity. In contrast, the mutants Q367D, N430D, L443E had no or only minor effects on Ib properties, while T360I, T360A and T360W caused a dramatic effect on ion selectivity and single-channel conductance, indicating gross disturbance of the oligomer structure. This suggests that, at least in the iota toxin family, T360 has a structural role in the pore organization. Moreover, introduction of charged residues within the channel (S339E-S341E) or in the vestibule (Q367D, N430D and L443E) had virtually no effect on chloroquine or Ia binding, whereas F454A, T360I, T360A and T360W strongly decreased the chloroquine and Ia affinity to Ib. These results support that distinct residues within the vestibule interact with chloroquine and Ia or are responsible for channel structure, while the channel lining amino acids play a less important role.

Pore-forming activity of clostridial binary toxins

“…The double mutant S339E/S341E is as effective as Ib wild type to mediate the translocation of Ia indicating that charged residues in the Ib pore-forming domain are nor required for the channel activity [122].…”

“…was substituted by an histidine and the Ia translocation activity of the resulting Ib mutant was partially impaired by anti-His-tag antibodies [122]. Indeed, after heptamerization and insertion into membrane, the seven introduced histidines within the loop are in a His-tag-like distribution accessible to the antibodies applied on the trans side of the membrane.…”

“…Thereby, the tip of the Ib loop seems to be critical in the control of the channel establishment or channel activity [122]. as wild type Ib [122].…”

“…Thereby, the tip of the Ib loop seems to be critical in the control of the channel establishment or channel activity [122]. as wild type Ib [122]. It seems that the hydrophobicity at the interface between the channel and the cis side vestibule is critical for the structural and functional organization of the channel.…”

“…F427 is also involved in prepore to functional pore conversion [85,123,124]. and F454 in Ib (F454A) result in strong alteration of the channel activity [122,125]. Thereby, the Ib single channel conductance is strongly increased (500 pS for F454A versus 85 pS for the wild type in 1 M KCl) and is unable to transport Ia [122].…”