Sizing the radius of the pore formed in erythrocytes and lipid vesicles bythe toxin sticholysin I from the sea anemone Stichodactyla helianthus

The radius of the pore formed by sticholysin I and II (StI, StII) in erythr ocytes and sticholysin I in lipid vesicles was investigated. The rate of co lloid osmotic lysis of human erythrocytes, exposed to one of the toxins in the presence of sugars of different size, was measured. The relative permea bility of each sugar was derived and the pore radius estimated with the Ren kin equation. The radius was similar for sticholysin I and It and was indep endent of the reference sugar chosen and of the toxin concentration applied . It was also the same when erythrocytes were pretreated with different tox in doses in the presence of a polyethylene glycol (PEG) large enough to pre vent lysis and thereafter transferred to solutions containing oligosacchari des of different size where they did lyse at different rates. The osmometri c behavior of large unilamellar vesicles (LUV) was thereafter used to estim ate the toxin lesion radius in a model system. LUV transferred to a hyperos motic solution with a certain sugar immediately shrank and then re-swelled at a rate dependent on the bilayer permeability to water and sugar. When LU V were previously permeabilized with StI, only a fraction of them, namely t hose not carrying pores, continued to behave as osmometers. By increasing t he size of the added sugar and approaching the pore radius, the fraction of osmometric LUV increased. Relative permeabilities were derived and used to estimate a channel radius around 1.2 nm, both for sugars and for PEGs. In conclusion the sticholysin pore has a constant size independent of toxin co ncentration and similar in natural and artificial membranes, suggesting it has a fixed predominant structure.