Disruption of the open conductance in the β-tongue mutants of Cytolysin A

Cytolysin A (ClyA) is a water-soluble alpha pore-forming toxin that assembles to form an oligomeric pore on host cell membranes. The ClyA monomer possesses an alpha-helical bundle with a beta-sheet subdomain (the beta-tongue) previously believed to be critical for pore assembly and/or insertion. Oligomerization of ClyA pores transforms the beta-tongue into a helix-turn-helix that embeds into the lipid bilayer. Here, we show that mutations of the beta-tongue did not prevent oligomerization or transmembrane insertion. Instead, beta-tongue substitution mutants yielded pores with decreased conductance while a deletion mutation resulted in pores that rapidly closed following membrane association. Our results suggest that the beta-tongue may play an essential structural role in stabilizing the open conformation of the transmembrane domain.