The γ Subunit of the B Cell Antigen-Receptor Complex is a C-Terminally Truncated Product of the B29 Gene

The predominant Ag-receptor complex of B cells consists of mlgM or mIgD noncovalently associated with glycosylated heterodimers of Ig-α and Ig-β or Ig-α and g-γ. Upon B cell stimulation the associated proteins are phosphorylated, giving rise to pp32/33 (α), pp37 (β), and pp34 (previously designated γ). Ig-α and Ig-β contain extended cytoplasmic structure (61 and 48 amino acids, respectively) and associate with cytoplasmic effectors indicating that they are directly involved in signal transduction. Here we report analysis of the structural relationship of mIgM- and mIgD-associated Ig-β and Ig-γ chains from mice. N-terminal sequence, immunoblotting, and physicochemical analyses show that both Ig-β and Ig-γ are products of the B cell-specific B29 gene and demonstrate that the 37-kDa Ig-β protein is the full length predicted product of the B29 gene. The Ig-associated protein that migrates in the 34-kDa range is actually two distinct species. The minor species is a phosphorylatable and underglycosylated form of full length Ig-β, and the major species is a C-terminally truncated form of B29, which we now designate Ig-γ. This conclusion is based on the observations that Ig-γ is composed of a core protein which is 3 to 4 kDa smaller than deglycosylated Ig-β, it is not phosphorylated, unlike Ig-β, and it does not react with an antiserum raised against a peptide of the seven C-terminal amino acids of B29. Based on these findings we estimate that Ig-γ is truncated by about 30 to 36 amino acid residues and hypothesize that the most 3′ B29 exon, which encodes the 32 C-terminal residues, may not be expressed in Ig-γ. All of the documented B29 products are found in association with both mIgM and mIgD. Interestingly, Ig-γ is found in intermediate and low density splenic B cells, but is not detectable in resting B cells. This raises the possibility that it may confer some distinct signaling function on the Ag receptors of these cells. Journal of Immunology, 1993, 150: 2814.