Structural parameters ofthemyelin transmembrane proteolipid inreverse micelles

TheFolch-Pi proteolipid is themostabundant structural protein fromthecentral nervoussystemmy- elin. Thisprotein-lipid complex, nor- mally insoluble inwater, requires only a small amount ofwater forsolubilization inreversemicelles ofsodiumbis(2- ethylhexyl) sulfosuccinate (AOT)in isooctane. Thecharacterization ofthe proteolipid-free and proteolipid-con- taining micelles was undertaken bylight scattering andfluorescence recovery afterfringe pattern photobleaching (FRAPP)experiments. Quasielastic light scattering (QELS) was carried out atahigh(200mM)AOTconcentration, atlowwater-to-surfactant moleratio (w, = 7)andatincreasing protein occupancy.Two apparent hydrody- namicradii, differing tenfold insize, were obtained fromcorrelation func- tions. Thesmaller one (RH= 5.2nm) remains constant andcorresponds to that measured forprotein-free micelles. Thelarger one increases linearly with protein concentration. Incontrast, FRAPPmeasurements ofself-diffusion coefficients were foundunaffected by theproteolipid concentration. Accord- ingly, theyhavebeenperformed at constant protein/surfactant molera- tios. Theequivalent RH,extrapolated to zero AOT concentration forprotein- free reversemicelles (2.9nm)andin thepresenceoftheproteolipid (4.6 nm),donotreveal themodeoforgani- zation previously suggested byQELS measurements. Thecomplexpicture emerging fromthis workrepresents a first stepinthecharacterization ofan integral membraneprotein inreverse micelles. 949~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 0006-3495/89/05/949/07 $2.00