Trapping of Vibrio cholerae Cytolysin in the Membrane-bound Monomeric State Blocks Membrane Insertion and Functional Pore Formation by the Toxin [Protein Structure and Folding]

May 2nd, 2014 by Rai, A. K., Chattopadhyay, K.

Vibrio cholerae cytolysin (VCC) is a potent membrane-damaging cytolytic toxin that belongs to the family of β-barrel pore-forming protein toxins (β-PFTs). VCC induces lysis of its target eukaryotic cells by forming transmembrane oligomeric β-barrel pores. Mechanism of membrane pore formation by VCC follows overall scheme of the archetypical β-PFT mode of action, in which water-soluble monomeric form of the toxin first binds to the target cell membrane, then assembles into a pre-pore oligomeric intermediate, and finally converts into the functional transmembrane oligomeric β-barrel pore. However, there exists a vast lacuna in our understanding regarding the intricate details of the membrane pore-formation process employed by VCC. In particular, membrane oligomerization and membrane insertion steps of the process have been described only to a limited extent. In the present study, we have determined the key residue(s) in VCC that are critical to trigger membrane oligomerization of the toxin. Alteration of such key residue(s) traps the toxin in its membrane-bound monomeric state, and abrogates subsequent oligomerization, membrane insertion, and functional transmembrane pore-formation events. Results obtained from our study also suggest that the membrane insertion of VCC depends critically on the oligomerization process, and it cannot be initiated in the membrane-bound monomeric form of the toxin. In sum, our study for the first time dissects membrane binding from the subsequent oligomerization and membrane insertion steps, and thus defines the exact sequence of events in the course of membrane pore formation process by VCC.
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