Membrane immunoglobulins are stabilized by interchain disulfide bonds occurring within the extracellular membrane-proximal domain

Membrane-bound immunoglobulins have, in addition to the transmembrane and cytoplasmic portions, an extracellular membrane-proximal domain (EMPD), absent in the secretory forms. EMPDs of immunoglobulin isotypes α, γ, and ε contain cysteines whose role has so far not been elucidated. Using a genetic strategy, we investigated the ability of these cysteines to form disulfide bridges. Shortened versions of human membrane immunoglobulins, depleted of cysteines known to form intermolecular disulfide bonds, were constructed and expressed on the surface of a B-cell line. The resulting membrane proteins contain a single chain fragment of variable regions (scFv) linked to the dimerizing domain from the immunoglobulin heavy chains (CH3 for α and γ or CH4 for ε isotypes), followed by the corresponding EMPD and the transmembrane and cytoplasmic domains. The two functional membrane versions of the ε chain, containing the short and long EMPD, were analyzed. Our results show that the single cysteine within α1L and γ1 EMPD and the short version of ε EMPD form an interchain disulfide bond. Conversely, the cysteine resident in the ε transmembrane domain remains unreacted. ε-long EMPD contains four cysteines; two are involved in interchain bonds while the remaining two are likely forming an intrachain bridge. Expression of a full-length membrane ε heavy chain mutant, in which Cys121 and Cys209 within domain CH2 (involved in interchain bridges) were mutated to alanines, confirmed that, within the complete IgE, EMPD cysteines form interchain disulfide bonds. In conclusion, we unveil evidence for additional covalent stabilization of membrane-bound immunoglobulins.