FIGURE 11-20 Production of membrane and secreted |i chains in B lymphocytes. Alternative processing of a primary RNA transcript results in the formation of mRNA for the membrane or secreted form of the |i heavy chain. B cell differentiation results in an increasing fraction of the | protein produced as the secreted form. TP, TM, and CY refer to tail piece, transmembrane, and cytoplasmic segments, respectively.
membrane C|4 is followed by a short spacer, 26 hydro-phobic transmembrane residues, and a cytoplasmic tail of three amino acids (lysine, valine, and lysine). In secreted IgM, on the other hand, the C|4 domain is followed by a tail piece containing polar amino acids. This transition from membrane to secreted Ig is caused by altered RNA processing of the heavy chain messenger RNA (mRNA). The primary RNA transcript in all IgM-producing B cells contains the rearranged VDJ cassette, the four C| exons coding for the constant (C) region domains, and the two exons encoding the transmembrane and cytoplasmic domains. Alternative processing of this transcript, which is regulated by RNA cleavage and the choice of polyadenylation sites, determines whether or not the transmembrane and cytoplasmic exons are included in the mature mRNA. If they are included, the | chain produced contains the amino acids that make up the transmembrane and cytoplasmic segments and is therefore anchored in the lipid bilayer of the plasma membrane. If, on the other hand, the transmembrane segment is excluded from the | chain, the carboxyl terminus consists of about 20 amino acids constituting the tail piece. Because this protein does not have a stretch of hydrophobic amino acids or a positively charged cyto-plasmic tail, it cannot remain anchored in the endoplas-mic reticulum membrane, resides initially in the luminal space of the secretory pathway, and is secreted. Thus, each B cell can synthesize both membrane and secreted Ig. Most of the Ig heavy chain mRNA in a plasma cell is cleaved at the upstream polyadenylation site, so most of this mRNA is of the secretory form. All CH genes contain similar membrane exons, and all heavy chains can be potentially expressed in membrane-bound and secreted forms. The secretory form of the 8 heavy chain is rarely made, however, so that IgD is usually present only as a membrane-bound protein.
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