Insertion of diphtheria toxin in lipid bilayers studied by spin label ESR

The pH dependence of the insertion of diphtheria toxin into bilayers of dioleoylphosphatidylglycerol (DOPG) has been studied by using ESR spectroscopy of spin-labeled phosphatidylglycerol with the reporter group at either the 5-position or the 14-position of the sn-2 chain (5-PGSL and 14-PGSL, respectively). At neutral pH, addition of diphtheria toxin has little effect on the ESR spectra of either spin label in large unilamellar vesicles of DOPG. At acidic pH, the outer hyperfine splitting of the 5-PGSL label is increased, and a second component corresponding to lipids whose chain motion is selectively restricted appears in the spectra of the 14-PGSL label, in the presence of diphtheria toxin. The motionally restricted component of 14-PGSL has a large outer hyperfine splitting (2Amax approximately 61 G) and corresponds to spin-labeled lipids the chains of which are in direct contact with the membrane-penetrant part of the inserted toxin. This restricted component is present, although to a lesser extent, in vesicles containing 90% of the zwitterionic lipid dioleoylphosphatidylcholine and displays a limited selectivity for negatively charged relative to zwitterionic spin-labeled phospholipids. The fraction of lipids which are motionally restricted by the toxin increases with decreasing pH, titrating in DOPG vesicles with an apparent pKa of approximately 6.1. The outer hyperfine splitting of the 5-PGSL label titrates with an apparent pKa of approximately 5.5, suggesting that this might be preferentially sensitive to a later stage in the insertion of the toxin.(ABSTRACT TRUNCATED AT 250 WORDS)