Understanding the biosynthesis of human IgMs through a combinatorial expression of mutant subunits that affect different assembly steps

Polymeric IgMs are secreted from plasma cells abundantly despite their structural complexity and intricate multimerization steps. To gain new insights into IgMs assembly mechanics that underwrite the high-level secretion, we characterized the biosynthetic process of a natural human IgM, SAM-6, using a recombinant HEK293 cell system. By creating a series of mutant subunits that differentially disrupt specific sets of inter-chain disulfide bonds, we assessed their effects on various aspects of IgM biosynthesis in 48 different mutant subunit combinations. The analysis included the visualization of intracellular biosynthetic events such as steady-state subcellular subunit distribution, secretory trafficking bottlenecks, and the ER-associated Russell body formation by fluorescent microscopy. We also characterized various extracellular events including secreted IgM product quality, secretion output, and the release of various assembly intermediates using biochemical and biophysical assays. In this combinatorial mutagenesis approach, we unexpectedly found that the loss of multiple inter-chain disulfide bonds, including the one between HC and {lambda}LC subunits, was tolerated in polymeric IgM formation and secretion. This finding revealed the vital role of underlying non-covalent protein-protein association not only during the orchestration of initial subunit interactions but also in maintaining the polymeric IgM product integrity during ER quality control steps, secretory pathway trafficking, and secretion. We suggest that the IgM assembly process is inherently robust and has a stopgap that permits the secretion of polymeric IgM even when not all the prescribed inter-chain disulfide bonds are formed. This study holistically presents the requirements and exemptions in polymeric IgM biosynthesis by encompassing the characterization of intracellular and extracellular events and the roles of covalent and non-covalent interactions. These findings can guide antibody engineering strategy when designing IgM-based multivalent modalities.