Biochimica et Biophysica Acta, 692 (1982) 339-344 339 Elsevier Biomedical Press BBA 71381 MICELLAR GANGLIOSIDES MEDIATE THE LIPID INSERTION OF CHOLERA TOXIN PROTOMER A M. TOMASI a, G. D'AGNOLO a and C. MONTECUCCO b Laboratorio di Biologia Cellulare e lmmunologia, lstituto superiore di sanit& Viale Regina Elena 299, 00161 Roma and h Centro C.N.R. per la Fisiologia dei Mitocondri e Laboratorio di Biologia e Patologia Molecolare, lstituto di Patologia Generale, Universitd di Padova, Via Loredan 16, Padova (ltaly) (Received June 18th, 1982) Key words: Cholera toxin; Ganglioside; Micelle; Detergent; Photolabeling The topology of the interaction of cholera toxin with ganglioside and detergent micelles was studied with the technique of hydrophobic photolabelling. Cholera toxin a and ,/polypeptide chains appear to penetrate into the hydrophobic core of ganglioside micelles. Micelles of SDS cause the labelling also of the ~ polypeptide chains, while Triton X-lO0 micelles have little ability to mediate the labelling of the toxin. The specific reduction of the a-v disulfide bond allows the penetration of the a polypeptide chain into Triton X-IO0 micelles, but does not affect the interaction of cholera toxin with either ganglioside or SDS micelles. Thus, ganglioside micelles appear to cause a conformational change of the native toxin, such as to induce the penetration of the a chain into the micelle hydrophobic core. Introduction Several intoxication phenomena involve the crossing of the plasma membrane by a protein toxic subunit [1]. The trigger of the process is considered to be the binding of the toxin to the saccharide component, either of a glycoprotein or of a glycohpid, on the external surface of the cell membrane. However, the molecular steps which allow the water-soluble toxin to cross the lipid barrier are poorly understood. On the basis of nuclear magnetic resonance and electron microscopy studies, Cullis and De Kruijff [2] have suggested that membrane processes such Abbreviations: GMI , galactosyl-N-acetylgalactosaminyl[N- acetylneuraminyl]galactosylglucosylceramide; GDla ' N-acetyl- neuraminylgalactosyl-N-acetylgalactosaminyl[N_acetyineuram. inyl]galactosylglucosylceramide; azidoglycolipid, 12-amino- N(2"nitr°'4-azidophenyl)dodecanoyl[6- 3H]glucosamine; SDS, sodium dodocyl sulfate. 0005-2736/82/0000-0000/$02.75 © 1982 Elsevier Biomedical Press as fusion and transport may be mediated by a transient destabilization of the lipid bilayer struc- ture. Freeze-facture experiments have shown that within a membrane there are microenvironments organized not in a bilayer but in a micellar-like structure [3]. Among natural lipids the ganglio- sides appear to be good candidates for the forma- tion of non-bilayer structures because of their large hydrophilic moiety [4,5]. This is reflected by the formation in water solutions of rnicelles of 250-450 kDa at concentrations lower than 10 -6 M [6,7]. Furthermore, even though the two hydro- carbon chains of gangliosides allow their insertion into the membrane, ganglioside concentrations higher than 12-16% with respect to total lipids are incompatible with a bilayer organization [8]. Thus, if gangliosides reach in selected regions of a bio- logical membrane a critical concentration the bi- layer may be destabilized. In fact, it has been shown that a modification of membrane fluidity is detectable when gangliosides are somehow segre-