Structural requirements for polymeric immunoglobulin assembly and association with J chain

Area of Science:

• Immunology
• Molecular Biology
• Protein Chemistry

Background:

• Immunoglobulins M (IgM) and A (IgA) exist as polymers, crucial for immune responses.
• Polymeric IgM typically forms pentamers with J chain, while IgA forms dimers with J chain.
• Both immunoglobulin types have a C-terminal tail-piece (tp) involved in polymerization via a cysteine residue.

Purpose of the Study:

• To investigate the structural requirements for IgM and IgA polymerization beyond the tail-piece.
• To determine the role of specific constant region domains in polymer assembly and J chain incorporation.

Main Methods:

• Analysis of chimeric immunoglobulins combining IgM and IgA domains.
• Characterization of polymer formation (dimers, pentamers, hexamers, higher-order polymers) and J chain presence.
• Assessment of N-linked carbohydrate processing on specific constructs.

Main Results:

• Polymerization restriction to dimers in IgA requires Cα3 and the IgA tail-piece (αtp), with Cα2 enhancing dimer assembly.
• IgM pentamer formation necessitates Cμ3, Cμ4, and the IgM tail-piece (μtp).
• Chimeric proteins showed varied polymerization states (hexamers, tetramers, high-order polymers) depending on domain composition, with altered J chain incorporation and heterogeneous glycosylation.

Conclusions:

• IgM and IgA polymer assembly is dictated by both the tail-piece and specific constant region domains.
• Constant region motifs, not just the tail-piece, are critical for determining polymer stoichiometry and J chain incorporation.
• Glycosylation patterns can be affected by domain composition during polymer formation.