Biosynthesis of membrane and secreted epsilon-chains during lipopolysaccharide-induced differentiation of an IgE+ murine B-lymphoma.

A switch variant of the I.29 murine B-cell lymphoma expressing membrane IgE and inducible by lipopolysaccharide (LPS) to increase the rate of IgE secretion was characterized. The cells (I.29 epsilon +2) express membrane-bound IgE, and also secrete considerable amounts of IgE when grown in regular culture medium. Membrane and secreted IgE contain structurally different heavy chains. The former is constituted by a 93-kd molecule (epsilon m), while secretory chains (epsilon s) have an apparent mol. wt of 86,000. Both epsilon m and epsilon s are heavily glycosylated: in the presence of tunicamycin their apparent mol. wt is reduced by approx. 35% (61 kd for epsilon m and 56 kd for epsilon s). Glycosylation is necessary for membrane expression and for secretion of IgE molecules. Stimulation with LPS leads to the disappearance of IgE molecules from the cell surface (determined by radioiodination) although epsilon m-chains are still synthesized, suggesting a defective transport of membrane IgE in LPS-treated cells. The epsilon m:epsilon s ratio decreases upon LPS stimulation. A similar change can be observed in the messenger RNAs specific for epsilon m and epsilon s, possibly suggesting a major pretranslational control for epsilon m and epsilon s biosynthesis.