Association of the receptor for immunoglobulin E with an endogenous polypeptide on rat basophilic leukemia cells

Enzymatic cleavage products of the alpha subunit of the receptor for immunoglobulin E

The high-affinity membrane receptor for immunoglobulin E on mast cells and on a tumor analogue, rat basophilic leukemia cells, consists of two polypeptide chains: an alpha chain of Mr congruent to 50,000 and a beta chain of Mr congruent to 30,000. In this study we reacted alpha chains purified from tumor cells with proteolytic and glycolytic enzymes and compared the products by using differential labeling procedures. A variety of proteolytic enzymes cleave the chain into two similar-sized fragments, alpha 1 and alpha 2. The alpha 1 fragment behaves as if it were slightly larger on electrophoresis through polyacrylamide gels and is rich in carbohydrate as determined by incorporation of [14C]glucosamine. Less incorporation is observed into alpha 2 but it, like alpha 1, binds to concanavalin A. Labeling of the surface proteins on intact cells by lactoperoxidase-catalyzed iodination leads to preferential, perhaps exclusive, labeling of the alpha 2 fragment. The relative proportion of incorporated 3H-labeled amino acids and radioactive Bolton--Hunter reagent suggests that the polypeptide portions of alpha 1 and alpha 2 are similar in size. From these and other data we propose that the alpha chain may be U-shaped. Results from endoglycosidase digestions show that the receptor as isolated is heterogeneous because of variable glycosylation.

IgE-induced histamine release from rat basophilic leukemia cell lines: isolation of releasing and nonreleasing clones

The rat basophilic leukemia (RBL) cell lines were cloned and the various sublines compared for their chromosome number, IgE-mediated histamine release and for IgE surface receptors. It was found that cell lines started from tumors at different times vary in both their chromosome number and their ability to release histamine by an IgE-mediated reaction. RBL-I and III have approximately 44 chromosomes and did not respond to an IgE-mediated reaction. RBL-II and RBL-IV have 68-73 chromosomes and showed moderate levels of histamine release (percent release mean = 5 +/- 2 and 10 +/- 4, respectively). The cloning of the RBL-IV line resulted in some sublines which were excellent histamine releasers (range 39-100%) and some which were relatively refractory (less than 10%) to IgE-mediated histamine release. These clones did not differ significantly in chromosome number. Recloning the releasing lines gave rise to poor releasers, whereas the recloning of poor releasers did not produce good releasers indicating that the mutational drift in culture is toward loss of histamine-releasing capacity. The number of IgE receptors and the rate of IgE association and dissociation were similar for the different cell lines. The study failed to disclose significant molecular weight differences in the IgE receptor from the various clones and sublines indicating that the failure to release probably does not reside in the receptor. The various cloned sublines are phenotypically stable, and the isolation of excellent histamine-releasing sublines are useful for studies of the complex phenomenon of the histamine release.

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).