Association of the Receptor for Immunoglobulin with an Endogenous Polypeptide on Rat Basophilic Leukemia Cells

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Abstract
Chemical crosslinking studies have been carried out to investigate the structural organization of the receptor for immunoglobulin E (IgE) on rat basophilic leukemia cells both before and after solubilization of the receptor with non-ionic detergent. A previously unidentified polypeptide with an apparent molecular weight of 30,000 -35,000 was found to crosslink to the IgE-binding polypeptide in a |:| stoichiometry when either whole cells or soluble cell extract were crosslinked at 4°C with dimethyl suberimidate or its cleavable analog, dimethyl 3,3′-dithiobis-propionimidate (DTBP). This polypeptide can be biosynthetically labeled with a mixture of 3H-amino acids but is not surface radioiodinated by the lactoperoxidase method. A biosynthetically labeled polypeptide of the same size copurifies with the uncrosslinked IgE-binding polypeptide to a variable extent, and this component is probably identical to that which crosslinks to the receptor. Bound IgE is crosslinked to the IgE-binding polypeptide by dimethyl suberimidate or DTBP, and this crosslinking can be prevented by amidinating the accessible amino groups on IgE with methyl acetimidate. Crosslinking of the IgE-binding polypeptide to the 30–35,000 molecular weight endogenous polypeptide is not affected by the presence of bound amidinated IgE. Likewise, triggering the cells to degranulate with dimeric or trimeric amidinated IgE prior to crosslinking of whole cells or soluble cell extract does not qualitatively affect this result. No evidence for new molecular associations which might occur as a result of the triggering signal was found in any of these studies. The results presented are discussed in relation to possible mechanisms by which the receptor for IgE mediates the triggering of the secretory process in mast cells and basophils.