CD79a and CD79b, two related proteins serve as the signalling subunits of the BcR .
They are type I transmembrane (TM) proteins, carrying an extracellular Ig-like domain, a transmembrane region with three polar amino acids, and a cytoplasmic tail with an ITAM motif.
The Ig domain of CD79a and/or CD79b binds to the membrane-proximal Ig domain of the mIg molecule. In addition, the TM region of the mIg heavy chains most likely crosses the membrane as an α -helix. Among these (25) amino acids, nine are polar in the case of mu chain, and seven in the case of delta chain.
These polar amino acids may be involved in protein-protein interactions in the membrane. One side of the helix is conserved between the different mIg classes and in the case of mIgM, it has been shown to bind to CD79a-CD79b. This side contains the conserved antigen receptor TM (CART) motif which is even conserved in the TcR α β, γ, and δ chains .
The other side of the alpha-helix binds to the BAP proteins in the case of mIgD . The cytoplasmic parts of the mIgM and mID are identical, and consist of the 3 amino acids lysine, valine, lysine. Apart from glycosylation differences in the CD79a protein, all mIg classes are non-covalently coupled with the same CD79a-CD79b heterodimer.
The extracellular spacer of the mIg heavy chain determines the glycosylation pattern of the CD79a protein . The assembly with the CD79a-CD79b heterodimer is a prerequisite for the transport of the mIg molecules from the endoplasmic reticulum (ER) to the cell surface . On the surface of the mature B cells, the IgD-BcR is 5-10 fold more abundantly expressed than the IgM-BcR. The delta TM region is responsible for the more stable association of mIgD with the CD79a-CD79b heterodimer .