Expression and plasma membrane localization of the mammalian B-cell receptor complex in transgenic Nicotiana tabacum

The B-cell antigen receptor (BCR), displayed on the plasma membrane of mature B cells of the mammalian immune system, is a multimeric complex consisting of a membrane-bound immunoglobulin (mIg) noncovalently associated with the Igα/Igβ heterodimer. In this study, we engineered transgenic tobacco plants expressing all four chains of the BCR. ELISA, Western blotting and confocal microscopy demonstrated that the BCR was correctly assembled in plants, predominantly in the plasma membrane, and that the noncovalent link was detergent sensitive. This is the first example of a noncovalently assembled plasma membrane-retained heterologous receptor in plants. In B cells of the mammalian immune system, following antigen binding to mIg, BCR is internalized and tyrosine residues on Igα and Igβ are phosphorylated activating a signaling cascade through interaction with protein kinases that ultimately leads to the initiation of gene expression. Expression of the BCR may therefore be an important tool for the study of plant endocytosis and the identification of previously unknown plant tyrosine kinases. The specificity and diversity of the antibody repertoire, coupled to the signal transduction capability of the Igα/Igβ heterodimer, also indicates that plants expressing BCR may in future be developed as environmental biosensors.