Fig. 1. Expression of secreted immunoglobulin and its membrane form in the BCR complex.

A. Comparison of structures for the secreted Ab with the membrane anchored form of Ig in the BCR complex. The BCR complex is composed of two heavy chain subunits (HC_m, green) with a C-terminal membrane-spanning domain (yellow, small arrows); two light chains (LC, dark gray), and one heterodimer containing Igα and Igβ (Schamel and Reth, 2000). The major secreted form of Ab contains heavy chain subunits HC_s lacking the membrane domain (right) and ending in a short novel C-terminal domain (blue). A similar model may be drawn for all Ig isotypes and subtypes. B. Comparison of processing of heavy chain transcripts. Upward arrows: In immature and mature B-cells the heavy chain transcript is subjected to splicing that excludes RNA encoding 3′ e UTR₂ (untranslated region 2) and includes the membrane spanning region exons (m₁, m₂) and the 3′ UTR₁ to make mRNA_m encoding HC_m. Downward arrows: In Ab-secreting plasma cells the rate of heavy chain transcription is increased and majority of mature heavy chain transcripts are subjected to alternate upstream polyadenylation using 3′ UTR₂ to produce mRNA_s encoding HC_s. C. Western blots showing the membrane isovariant of heavy chain HC_m (yellow dot) and smaller secreted isovariant HC_s (blue dot) were present not only in a mouse B-lymphoma cell line (IgG2a expressing memory B-cell line A20.2J), but also in mouse plasma cell lines expressing various isotypes: IgG2a (A20); IgG2a (AxJ); IgG1 (anti-SC32); and IgG2b (4T001) (Milcarek et al., 1996). Twenty five μg of total protein (Bradford, 1976) from each cell extract was resolved by SDS-PAGE, blotted to membrane, and probed with rabbit antisera to the mouse heavy chain. The blots were processed as described previously(Kandasamy et al., 1999). The approximate sizes of the various heavy chain isovariants are indicated on the right.