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BCR assembly; cystein mutants; Ig-α/Ig-β heterodimer; Ig fold; surface mIgM expression
Abstract:
Structural information about immune receptor complexes is important for understanding signal transduction mechanisms. We have used the Drosophila S2 cell reconstitution system for identification of disulfide bonds within and between CD79a (Ig-α) and CD79b (Ig-β), the heterodimeric signal transducing element of the B cell antigen receptor (BCR). Cysteines 113 and 135 of Ig-α and Ig-β, respectively, form the intermolecular disulfide bridge stabilizing the
Ig-α/Ig-β heterodimer in both S2 cells and the B cell line J558L. Furthermore, using transfected S2 cells, two putative intramolecular disulfide bonds in the Ig-like domain of Ig-β were identified. Ig-βC65 and Ig-βC120 form the canonical Ig fold disulfide bond. In addition, Ig-βC43 and Ig-βC124 also bind covalently. Individual cysteine to serine mutations in Ig-α had no influence on membrane-bound Ig (mIg)-M expression on the surface of S2 cells. In contrast, mIgM expression on the surface of B cells expressing Ig-αC113S was reduced, indicating that this intermolecular bond is prerequisite for efficient IgM-BCR formation. Our data also suggest that the Ig-α/Ig-β heterodimer can assemble into oligomers.
