title: The effects of lipids on the structure of the eukaryotic cytolysin equinatoxin II: a synchrotron radiation circular dichroism spectroscopic study.
abstract: Synchrotron radiation circular dichroism (SRCD) spectroscopy studies of the eukaryotic pore-forming protein equinatoxin II (EqtII) were carried out in solution and in the presence of micelles or small unilamellar vesicles (SUV) of different lipid composition. The SRCD structural data was correlated with calcein leakage from SUV and with partitioning of EqtII to liposomes, and micelles, according to haemolysis assays. The structure of EqtII in water and dodecylphosphocholine micelles as determined by SRCD was similar to the values calculated from crystal and solution structures of the protein, and no changes were observed with the addition of sphingomyelin (SM). SM is required to trigger pore formation in biological and model membranes, but our results suggest that SM alone is not sufficient to trigger dissociation of the N-terminal helix and further structural rearrangements required to produce a pore. Significant changes in conformation of EqtII were detected with unsaturated phospholipid (DOPC) vesicles when SM was added, but not with saturated phospholipids (DMPC), which suggests that not only is membrane curvature important, but also the fluidity of the bilayer. The SRCD data indicated that the EqtII structure in the presence of DOPC:SM SUV represents the 'bound' state and the 'free' state is represented by spectra for DOPC or DOPC:Chol vesicles, which correlates with the high lytic activity for SUV of DOPC:SM. The SRCD results provide insight into the lipid requirements for structural rearrangements associated with EqtII toxicity and lysis.
authors.affiliation: Department of Crystallography, Birkbeck College, University of London, London WC1E 7HX, UK.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Miles
authors.forename: Alison
authors.initials: A
authors.lastname: Drechsler
authors.forename: Katarina
authors.initials: K
authors.lastname: Kristan
authors.forename: Gregor
authors.initials: G
authors.lastname: Anderluh
authors.forename: Raymond S
authors.initials: RS
authors.lastname: Norton
authors.forename: B A
authors.initials: BA
authors.lastname: Wallace
authors.forename: Frances
authors.initials: F
authors.lastname: Separovic
mesh_terms: D000818:Animals; D002942:Circular Dichroism / Q000379:methods*; D003064:Cnidarian Venoms / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry; D008055:Lipids / Q000737:chemistry*; D008823:Micelles; D011487:Protein Conformation; D017356:Synchrotrons; D053835:Unilamellar Liposomes / Q000737:chemistry; D014662:Vasoconstrictor Agents / Q000737:chemistry
chemical_list: D003064:Cnidarian Venoms; D008051:Lipid Bilayers; D008055:Lipids; D008823:Micelles; D053835:Unilamellar Liposomes; D014662:Vasoconstrictor Agents; C012307:equinatoxin