Thiol-reducing agents and calcium perturbants alter intracellular sorting of immunoglobulin M

The secretory form of IgM (sIgM) undergoes developmentally regulated intracellular sorting and transport, as demonstrated by the distinct assembly patterns and intracellular fate exhibited by its μ heavy chain (μs). In the 38C B lymphocytes, μs-containing monomers are retained and degraded intracellularly, whereas in the 38C-derived D2 hybridoma, μs-containing polymers are secreted. Here we show that sorting of sIgM is impaired in the presence of the thiol-reducing agent β-mercaptoethanol or when cellular calcium sequestration is perturbed either with the Ca²⁺ ionophore A23187 or with thapsigargin, an inhibitor of endoplasmic reticulum Ca²⁺-ATPase. Under these conditions, sIgM evades retention/degradation in the 38C cells, resulting in its enhanced secretion. Conversely, in the D2 cells, secretion is strongly attenuated, and sIgM is partly degraded. Both cell types secrete unusually processed sIgM, which is completely resistant to endoglycosidase H, unlike the partially sensitive sIgM secreted in the usual manner, indicating terminal glycosylation of all carbohydrate moieties of μs. Moreover, secretion is no longer restricted to fully assembled polymeric sIgM, and in both cell types under Ca²⁺ perturbation preferentially hemimeric and monomeric assembly intermediates are detected in the medium. Our results suggest that the intracellular fate of sIgM is collectively determined by several sorting events that confer sIgM retention/degradation, ensure sIgM secretion from D2 cells, and prevent the arrival of assembly intermediates to the cell surface. All of these sorting events appear to require high Ca²⁺ concentration and oxidizing thiol redox state, the conditions that prevail in the lumen of the endoplasmic reticulum. Thus, thiol and calcium are implicated as modulators of intracellular traffic.