Introduction

Significance and Background

History and Discovery

Structure

Transmembrane Region

The IgM BCR is tethered to B-cell membranes through the which is broken up into both extracellular and integral domains. IgM BCR assembly requires dimerization of the Ig alpha and Ig beta subunits which embed within the B-cell membrane. The is dimerized in both the integral region, with a hydrogen bond, and within the extracellular region with a . Although the mechanism of disulfide bridge formation is still unknown, it is believed that via N-linked glycosylation (NAGs, shown in green) of various asparagine residues in the extracellular region of both the alpha and Ig beta chains help to facilitate this process. Chaperon proteins remain bound to the alpha and beta subunits until both dimerizations occur at which point the rest of the BCR complex can be recruited.

After Ig alpha/Ig beta dimerization the transmembrane helices of the heavy chains can embed within the B-cell membrane as well. The side chains of this are primarily hydrophobic side chains that allow for interactions with the hydrophobic tails in the phospholipid bilayer. A total of 9 polar residues (picture that zooms in here??) between each of the heavy chains are included on the interior of the helix structure which interact with a few polar residues on the Ig alpha and Ig beta chains to hold the complex together. Additional interactions between the Ig alpha/Ig beta dimer and the heavy chains occur in the constant region.

Fc Region

Fab Region

Because the Fab region of IgM is so poorly resolved, a structural comparison to another antibody was performed to approximate where an antigen would bind to the . Figure 1 on the left shows the antigen binding motif (located on the Fab region) of an Ig-BCR complex that was engineered to contain the variable region of a neutralizing antibody called VCR01, an antibody that targets the epitope of HIV molecules. It contains areas referred to as complementary-determining regions, or CDRs, which are where the antigen makes contact with the antibody on the Fab domain. Showing them as surface representation allows us to make structural comparisons to the IgM antibody, and to highlight their similarities. The CDRs are similarly placed within the heavy and light chain variable regions between both antibodies. It is speculated that they are structurally similar because the VCR01 antibody can effectively target multiple HIV strains while IgM is the preliminary antibody produced and released during early stages of the immune response, thus it is able to respond in larger concentrations while antibodies that are more specific to the antigen are being produced.

Figure 1. Surface Representation of IgM Antibody Binding Pocket

The Fab region of the IgM antibody is still poorly resolved, thus the specific side chain interactions between the heavy and light chains have not been determined. This depiction of the shows how the 4 β-sandwiches fit together; heavy chain A and B of the Fab region form a complex with the rest of the molecule via interactions with the heavy A and B of the Fc region, before continuing down into the intracellular domain to interact with the transmembrane region. The light chains however are only connected to the complex by forming interactions with the heavy chains within the Fab region. Although the specific residues within the Fab region have yet to be identified, it is estimated that each β-sandwich contains one disulfide bridge with additional hydrogen bonds.

Relevance

Signal Transduction

Figure 2. IgM Antibody Signal Transduction following Antigen Binding