The human Ig-beta cDNA sequence, a homologue of murine B29, is identical in B cell and plasma cell lines producing all the human Ig isotypes.

The B cell Ag receptor complex consists of at least two disulfide-linked, heterodimeric structures: the clonally restricted membrane Ig (mIg) molecule and the nonpolymorphic Ig-alpha:Ig-beta protein dimer. The latter molecule is encoded by two separate genes, mb-1 and B29. The DNA sequences of murine and human mb-1 and murine B29 have been determined previously. This study describes the sequence of the full-length human cDNA homologue of the murine Ig-beta/B29 message. The human sequence codes for a protein that displays the typical subunit features of a transmembrane member of the Ig superfamily. The transmembrane and intracytoplasmic domains exhibit striking nucleotide and amino acid sequence similarity between the two species. These regions show almost complete conservation of areas presumed to be involved in noncovalent interactions with other members of the receptor complex and with intracellular kinases and cytoskeletal components. The only sequence dissimilarity seen in these presumed critical areas involves the Y-E-G-L-N motif, a potential target for tyrosine phosphorylation. In contrast, the extracellular portion is much more divergent. Inasmuch as similar patterns of species diversity have been reported for Ig-alpha, the Ig-alpha and Ig-beta molecules may have coevolved to maintain species-specific extracellular interactions between one another and with mIg. Similar to the Ig-alpha molecule, the Ig-beta sequence is identical in B lineage cells expressing all five Ig isotypes. However, in contrast to the Ig-alpha molecule, the Ig-beta sequence is expressed at apparently similar levels in terminally differentiated, mIg- plasma cells as well as in mIg+, mature B cells. These data suggest that Ig-beta has functions in addition to those associated with surface mIg expression.