Characterization of interactions within the Igα/Igβ transmembrane domains of the human B-cell receptor provides insights into receptor assembly

The B-cell receptor (BCR), a complex comprised of a membrane-associated immunoglobulin and the Ig alpha/beta heterodimer, is one of the most important immune receptors in humans and controls B-cell development, activity, selection, and death. BCR signaling plays key roles in autoimmune diseases and lymphoproliferative disorders, yet, despite the clinical significance of this protein complex, key regions (i.e., the transmembrane domains) have yet to be structurally characterized. The mechanism for BCR signaling also remains unclear and has been variously described by the mutually exclusive cross-linking and dissociation activation models. Common to these models is the significance of local plasma membrane composition, which implies that interactions between BCR transmembrane domains (TMDs) play a role in receptor functionality. Here we used an in vivo assay of TMD oligomerization called GALLEX alongside spectroscopic and computational methods to characterize the structures and interactions of human Ig alpha and Ig beta TMDs in detergent micelles and natural membranes. We observed weak self-association of the Ig beta TMD and strong self-association of the Ig alpha TMD, which scanning mutagenesis revealed was entirely stabilized by an E-X-10-P motif. We also demonstrated strong heterotypic interactions between the Ig alpha and Ig beta TMDs both in vitro and in vivo, which scanning mutagenesis and computational models suggest is multiconfigurational but can accommodate distinct interaction sites for self-interactions and heterotypic interactions of the Ig alpha TMD. Taken together, these results demonstrate that the TMDs of the human BCR are sites of strong protein-protein interactions that may direct BCR assembly, endoplasmic reticulum retention, and immune signaling.