Polymeric nonelectrolytes to probe pore geometry: Application to the alpha-toxin transmembrane channel

Asymmetrical (one-sided) application of penetrating water-soluble polymers, polyethylene glycols (PEGs), to a well-defined channel formed by Staphyloc occus aureus oc-toxin is shown to probe channel pore geometry in more detai l than their symmetrical (two-sided) application. Polymers added to the cis side of the planar lipid membrane (the side of protein addition) affect ch annel conductance differently than polymers added to the trans side. Becaus e a satisfactory theory quantitatively describing PEG partitioning into a c hannel pore does not exist, we apply the simple empirical rules proposed pr eviously (Krasilnikov et al., 1998, J. Membr. Biol. 161:83-92) to gauge the size of pore openings as well as the size and position of constrictions al ong the pore axis. We estimate the radii of the two openings of the channel to be practically identical and equal to 1.2-1.3 nm. Two apparent constric tions with radii of similar to 0.9 nm and similar to 0.6-0.7 nm are inferre d to be present in the channel lumen, the larger one being closer to the ci s side. These structural findings agree well with crystallographic data on the channel structure (Song et al., 1996, Science. 274:1859-1866) and verif y the practicality of polymer probing. The general features of PEG partitio ning are examined using available theoretical considerations, assuming ther e is no attraction between PEG and the channel lumen. It is shown that the sharp dependence of the partition coefficient on polymer molecular weight f ound under both symmetrical and asymmetrical polymer application can be rat ionalized within a "hard sphere nonideal solution model." This finding is r ather surprising because PEG forms highly flexible coils in water with a Ku hn length of only several Angstroms.