Differential sensitivity of pneumolysin-induced channels to gating by divalent cations

The induction of channels across planar lipid bilayers by purified, recombinant pneumolysin (a hemolytic protein from Streptococcus pneumoniae) has been studied by measuring increases in electrical conductivity. Pneumolysin-induced channels exhibit a wide range of single channel conductances (less than 50 pS to greater than 1 nS at 0.1 M KCl). Channels can be categorized on the basis of their K+:Cl- selectivity: the smallest channels are strongly cation selective, with t+ (the cation transference number) approaching 1.0; the largest channels are unselective (t+ approximately 0.5). Channels tend to remain open at all voltages (-150 to 150 mV); only the smallest channels exhibit any rectification. In the presence of divalent cations (1-5 mM Zn2+; 10-20 mM Ca2+), small (less than 50 pS) and medium-sized (50 pS to 1 nS) channels are closed in a voltage-dependent manner (more closure at higher voltages); at 0 voltage channels reopen. Overall selectivity is reduced by divalent cations, compatible with small, selective channels being closed preferentially to large, nonselective ones. It is concluded that a single molecular species (pneumolysin) induces multiple-sized channels that can be categorized by cation:anion selectivity and by their sensitivity to closure by divalent cations.