EVIDENCE FOR SODIUM DODECYL SULFATE PROTEIN COMPLEXES ADOPTING A NECKLACE STRUCTURE/

Structural analysis by cryo-electron microscopy and small-angle X-ray scattering of ten sodium dodecyl sulfate/protein complexes in 25 mM Tr is/HCl, 0.192 M glycine, pH 8.3, showed necklace-like structures of sp herical micelles dispersed along the unfolded peptide chain. The micel les of most SDS/protein complexes had a constant diameter (approximate to 6.2 nm), slightly larger than purl SDS micelles (approximate to 5. 7 nm), all micelles possessing a degree of surface roughness. The mice lle-associated polypeptide is mostly situated at the interface of the sulfate head groups and hydrocarbon core, intruding into the core rath er than outward from the surface. Proteins with a molecular mass less than about 20 kDa formed complexes with a single SDS micelle. Multi-mi cellar SDS/protein complexes had centre-to-centre intermicellar distan ces in the range 7.0-12.0 nm. Our findings on the constancy of micella r size, number of micelles/complex, and the relationship between the d egree of occupancy of micelles and a polypeptide's molecular mass, hav e enabled us to speculate on the correlation between the electrophoret ic mobility of a polypeptide in SDS/PAGE and its molecular mass. The a nomalous electrophoretic behaviour observed for the sodium dodecyl sul fate/histone H5 complex is accounted for by the large micelle of its c omplex.