Abstract
From the numerous surface markers of a B lymphocyte, the B-cell antigen receptor (BCR) complex is probably the most powerful marker influencing the developmental processes of the cell. The BCR consists of the membrane-bound immunoglobulin (mIg) but, depending on the state of differentiation, may be associated with a couple of other transmembrane proteins, most notably Igα (CD79a) and Igβ (CD79b).1 The N-terminal end of the mIg harbours the antigen-binding site, characterized by an incredibly high potential for diversification and built up by the variable regions of heavy and light chains. Followed by three or four constant domains, depending on the selected immunoglobulin-isotype, the mIg finally expresses two further domains: a transmembrane domain and a cytoplasmic tail, both of which vary in their isotype-specific amino acid composition.2 So far, the sheath proteins Igα and Igβ are known as the signal transduction component of the BCR complex, connecting the antigen receptor to the tyrosine phosphorylation pathways in the cell. All isotypes of mIg can form a complex with Igα and Igβ,3 indicating an involvement of all isotypes in the signal transduction pathway. Venkitaraman et al.3 showed that for mIgG and mIgD the Igα/Igβ sheath is not required for surface expression. However, Igα/Igβ is the minimum requirement for signal transduction4 and, in the case of IgM, is responsible for the release from intracellular retention sites.5 Igα is expressed by the mb-1 gene and is a 32 000 MW glycoprotein. Igβ (B29 gene) can be expressed in two different isoforms of 37 000 and 39 000 MW, respectively. Interestingly, Igα can be differentially glycosylated. Pogue and Goodnow6 suggested that the extracellular spacer domain of mIgD is necessary and sufficient to confer the mIgD-specific glycosylation pattern of the mb-1 gene.6 However, it remains to be elucidated if and how alternative glycosylation of mb-1 may affect signalling competence or internalization.
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