Different species of messenger RNA encode receptor and secretory IgM mu chains differing at their carboxy termini.

Biosynthetic studies in the presence of an inhibitor of glycosylation indicate that individual human lymphoma-derived cell lines can synthesize both membrne receptor and presumptive secretory forms of IgM mu chains. The receptor form has a larger polypeptide chain than the secretory form and possesses a different C-terminus, but similar N-terminus, consistent with the presence of a C-terminal hydrophobic "tail" for integral membrane binding. Messenger RNA isolated from these cells directs the synthesis of both forms of mu chains in a wheat germ translation system, indicating the presence of independent mRNAs for each form. It is proposed that the synthetic pathways for receptor and secretory IgM diverge at the post-transcriptional level, possibly by differential RNA splicing to give mRNA molecules with or without a translatable "tail" segment.