The detection of associated erythrocyte membrane proteins by cross-linking and surface radiolabelling

Membrane proteins of intact erythrocytes were cross-linked using the cleavable homobifunctional reagent, 3,3'-dithiobis(propionimidate) and the non-cleavable heterobifunctional reagent, 4-azidobenzimidate. Subsequently, the exposed cross-linked membrane proteins were radiolabelled by the lactoperoxidase method. Cross-linked and radiolabelled membrane proteins were analyzed by two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis in which resolution in the second dimension was preceded by mercaptoethanol treatment. Complexes so formed by 3,3'-dithiobis(propionimidate) were cleavable, whilst components cross-linked by 4-azidobenzimidate were uncleavable. Three complexes were found. Two of these contained protein III as the only radiolabelled component. It was assumed that the first (Mr approx. 200000) represents a dimer and the second (Mr approx. 300000) an oligomer of protein III. The third complex, which was identified after cross-linking with 4-azidobenzimidate, consisted of PAS II and a low molecular weight (lipid-like) component. Since PAS I (dimer of a PAS II) was not associated with this component it was concluded that PAS I and PAS II are situated in different environments of the erythrocyte membrane.