Evidence that Nystatin Channels Form at the Boundaries, Not the Interiors of Lipid Domains

Nystatin (nys) is an antifungal agent that preferentially forms ion channels in membranes containing the sterol, ergosterol (erg). The structure of the nystatin channel is not clear, but it is known that multiple nystatin monomers must aggregate to form channels in a sterol-rich membrane. When nys/erg containing vesicles are fused to a sterol-free bilayer, characteristic spikelike changes in membrane conductance are observed. An abrupt increase in conductance is followed by a decay that is generally stepwise linear and the decay time depends strongly on [erg]. These data are inconsistent with the hypothesis that nys channels form uniformly throughout the membrane and decay independently (which would produce exponential decay). We propose that channels are located at the boundaries of lipid superlattices such that diffusion of erg out of the lattice results in correlated channel decay. This was tested using a statistical mechanical analysis and Monte Carlo simulations, which reveal details of the diffusion process and provide insight into conditions at superlattice boundaries during decay. This analysis predicts the linear decay schemes and the dramatic drop in channel decay time observed at erg mol % = 50. This interpretation also explains puzzling data relating conductance spike height to vesicle diameter.