Biophys J, December 1999, p. 3023-3033, Vol. 77, No. 6

Polymeric Nonelectrolytes to Probe Pore Geometry: Application to the -Toxin Transmembrane Channel

 ^*Laboratory of Membrane Biophysics, Department of Biophysics and Radiobiology, Federal University of Pernambuco, 50670-901, Recife, PE, Brazil;  ^#Laboratory of Molecular Physiology, Institute of Physiology and Biophysics, 700095 Tashkent, Uzbekistan;  ^§Laboratory of Physical and Structural Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892 USA; and  ^¶St. Petersburg Nuclear Physics Institute, Gatchina 188350, Russia

Asymmetrical (one-sided) application of penetrating water-soluble polymers, polyethylene glycols (PEGs), to a well-defined channel formed by Staphylococcus aureus -toxin is shown to probe channel pore geometry in more detail than their symmetrical (two-sided) application. Polymers added to the cis side of the planar lipid membrane (the side of protein addition) affect channel conductance differently than polymers added to the trans side. Because a satisfactory theory quantitatively describing PEG partitioning into a channel pore does not exist, we apply the simple empirical rules proposed previously (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 along 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 constrictions with radii of ~0.9 nm and ~0.6-0.7 nm are inferred to be present in the channel lumen, the larger one being closer to the cis side. These structural findings agree well with crystallographic data on the channel structure (Song et al., 1996, Science. 274:1859-1866) and verify the practicality of polymer probing. The general features of PEG partitioning are examined using available theoretical considerations, assuming there is no attraction between PEG and the channel lumen. It is shown that the sharp dependence of the partition coefficient on polymer molecular weight found under both symmetrical and asymmetrical polymer application can be rationalized within a "hard sphere nonideal solution model." This finding is rather surprising because PEG forms highly flexible coils in water with a Kuhn length of only several Angstroms.

Biophys J, December 1999, p. 3023-3033, Vol. 77, No. 6
© 1999 by the Biophysical Society   0006-3495/99/12/3023/11  $2.00

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