Reaction of 5-iodonaphthyl-1-nitrene with the IgE receptor on normal and tumour mast cells.

Mast cells, basophils and a tumour analogue--rat basophilic leukaemia (RBL) cells--have a surface glycoprotein (R epsilon) which specifically binds monomeric immunoglobulin E (IgE), and aggregation of R epsilon causes secretion. When isolated from non-ionic detergent extracts of surface-labelled RBL cells by IgE-specific immunoprecipitation R epsilon appears as a 50,000 (50 K) to 60,000 (60 K) molecular weight (MW) band on electrophoresis in polyacrylamide gels in SDS (SDS-PAGE). Likewise, only a 50 component is observed when the polypeptide that binds IgE is isolated by affinity chromatography in conditions which prevent aggregation of the IgE, even when intrinsically labelled R epsilon is studied. To determine how R epsilon is inserted into the plasma membrane, we reacted RBL cells with the photolysable hydrophobic reagent 5-iodonaphthyl-1-azide (INA), which preferentially labels the intramembranous segments of several intrinsic membrane proteins. We report here that, surprisingly, the label was found, not on the 50 K glycopeptide, but only on a 30 K component which other studies suggest is a subunit of R epsilon (ref 9, 12).