The BCell Receptor

Immunologists have long been puzzled about how mIg mediates an activating signal after contact with an antigen. The dilemma is that all isotypes of mIg have very short cytoplas-mic tails: the mIgM and mIgD cytoplasmic tails contain only 3 amino acids; the mIgA tail, 14 amino acids; and the mIgG and mIgE tails, 28 amino acids. In each case, the cytoplasmic tail is too short to be able to associate with intracellular signaling molecules (e.g., tyrosine kinases and G proteins).

The answer to this puzzle is that mIg does not constitute the entire antigen-binding receptor on B cells. Rather, the B-cell receptor (BCR) is a transmembrane protein complex composed of mIg and disulfide-linked heterodimers called Ig-a/Ig-p. Molecules of this heterodimer associate with an mIg molecule to form a BCR (Figure 4-18). The Ig-a chain mIg mIg

Cell Receptor

V V V V Vi A A A A Ai

1 V ¥ V V V g ¡ A A A A A i

1 A A/A A A A A

Plasma membrane

48-aa tail

Cytoplasmic tails

Plasma membrane

6l-aa tail

Cytoplasmic tails


General structure of the B-cell receptor (BCR). This antigen-binding receptor is composed of membrane-bound immunoglobulin (mIg) and disulfide-linked heterodimers called Ig-a/Ig-p. Each heterodimer contains the immunoglobulin-fold structure and cytoplasmic tails much longer than those of mIg. As depicted, an mIg molecule is associated with one Ig-a/Ig-P heterodimer. [Adapted from A. D. Keegan and W. E. Paul, 1992, Immunol. Today 13:63, and M. Reth, 1992, Annu. Rev. Immunol. 10:97.]

has a long cytoplasmic tail containing 61 amino acids; the tail of the Ig-p chain contains 48 amino acids. The tails in both Ig-a and Ig-p are long enough to interact with intracellular signaling molecules. Discovery of the Ig-a/Ig-p heterodimer by Michael Reth and his colleagues in the early 1990s has substantially furthered understanding of B-cell activation, which is discussed in detail in Chapter 11.

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