Introduction

Immunoglobulin M, or IgM, is one of multiple types of immunoglobulins that exist in humans. IgM presents itself on the surface of a B cell to act as a B Cell Receptor (BCR). Upon the binding of an antigen to the BCR, the B cell will activate, proliferate, and produce other Ig compounds. These include IgG, IgD, IgA, and IgE antibodies, which all have specific roles in the various forms of immune response. Because of this, activation of the IgM BCR is a critical step in the beginning of an immune response.

The structure of IgM was determined using Cryo-EM to visualize the atoms within the protein. However, due to poor resolution within specific regions of the IgM BCR, not every atom has been able to be visualized.

Structure

The IgM BCR consists of six separate chains (Figure 1) that make up three main domains in the molecule. A depiction of the IgM shows two heavy and two light chains together form the Fab region, or variable fragment at the top of the molecule where the antigen binding sites are located. The two heavy chains extend below the Fab region through the Fc region and eventually connect to the Igα/β heterodimer to form the transmembrane region which anchors the overall complex to the B cell. The overall structure, expression, and function of the IgM BCR has been found to be strongly influenced by the transmembrane region in which Ig α/β interactions as a heterodimer influence cell surface expression, receptor assembly, and effective signal transduction. In each domain, interactions between individual chains are important to understand the complex as a whole. All future 3D depictions will be as in Figure 1.

Figure 1. IgM BCR Structure Overview. Depiction of the IgM BCR expressed on the membrane of a B cell. Includes all major components including the α/β heterodimer, heavy and light chains, antigen binding sites, and the ITAM region for signal transduction.

Transmembrane Region

The IgM BCR is anchored to B-cell membranes through the which is broken up into both extracellular and integral domains which sit on top of or span through the membrane, respectively. IgM BCR assembly requires dimerization of the Igα and Igβ subunits which embed within the B-cell membrane. The dimerizes within the extracellular region with a . Additional dimerization is believed to occur within the integral region via a hydrogen bond; the involved residues and interaction have not been confirmed. Although the mechanism of disulfide bridge formation is still unknown, it is believed that via N-linked glycosylation (NAGs) on various asparagine residues in the extracellular region of both the Igα and and Igβ chains help facilitate this process. Chaperone proteins remain bound to the alpha and beta subunits until both dimerizations occur; at this point the rest of the BCR complex can be recruited.

After Igα and Igβ dimerization, the transmembrane helices of the heavy chains can embed within the B-cell membrane. The side chains of this are primarily hydrophobic side chains that allow for interactions with the hydrophobic tails in the phospholipid bilayer. The 4 helices (Figure 2) are primarily held together through hydrophobic interactions; however, a a few polar residues are included on the interior of the helix structure which interact with a few polar residues on the Igα and Igβ chains.

Furthermore, both the Igα and Igβ chains have cytoplasmic tails that extend into the B cell (Figure 1). Each of these tails contain an immuno-receptor tyrosine-based activation motif (ITAM) region to facilitate signal transduction (Figure 4).

Figure 2. 4-pass integral helix. Pymol image of the integral helices in IgM BCR (PDB:7xq8) rotated on the x and y axes. Side chains are shown as sticks. Brown=Ig alpha, orange=Ig beta, pink=heavy chain A, blue=heavy chain B.

Within the transmembrane region, heavy chains A and B associate asymmetrically to facilitate intracellular signaling cascades. The allows them to pack together via Van der Waal contacts, but there are also prominent hydrogen bonds between each chain. More specifically, the hydroxyl group from Ser584 on heavy chain A donates a hydrogen bond to Ser584 and to Ser588 on heavy chain B. This creates a bifurcated bond (blue link), essentially forming a “fork” between the two chains to help stabilize them and maintain the transmission of the signal once the cell is activated. Because transmembrane Ig molecules cannot efficiently initiate the signal cascade, they must associate with the Igα and Igβ proteins w/in the BCR.

Fc Region

The constant region of IgM is made up of the 2 . These heavy chains form a bridge to connect the Fab fragment, or variable region, to the transmembrane region. They also act as a wire to allow the variable region to send a cellular signal through to the intermembrane region once an antigen has been bound.

help to stabilize and hold the heavy chains and Ig Alpha/Beta chains together in the extracellular portion of the intermembrane region.

Because a conformational change occurs throughout the entirety of the IgM-BCR complex, the Fc region must be able to tolerate the contortion of the molecule as the antigen binds. In constant region two, which is located at the start of the Fc region, heavy chains A and B make a to stabilize the IgM-BCR and drive downstream signaling.

To maximize the Fc region’s signal transduction efficiency and Van der Waals contacts, constant region two of heavy chain A makes an asymmetrical association with constant region three of heavy chain B to create a . More specifically, Arg243 and Arg251 residues from heavy chain A donate three hydrogen bonds to Leu433, Thr431, and Asp376 residues on heavy chain B. Furthermore, Leu313 of heavy chain A accepts a hydrogen bond from Thr429 on heavy chain B.

Fab Region

The Fab region of the antibody is where antigen recognition occurs upon binding. On each arm is one heavy and one light chain, both containing domains identical to their respective counterparts. Repeats of β-sandwiches form the constant and variable domains (blue link) within the Fab region as antigen recognition occurs at the variable domain while the constant domain connects it to the rest of the IgM complex. Because the Fab region of IgM is poorly resolved, a structural analysis of an HIV neutralizing antibody called VCR01 was performed to approximate where an antigen would bind to at the . The IgM-BCR contains areas referred to as complementary-determining regions (blue link)(CDRs), which are where the antigen makes contact with the antibody on the Fab domain. Figure 2 depicts this as a surface representation given that the specific residues within the antigen-binding motif are unknown.

Due to the poor resolution of the Fab region, specific side chain interactions between the heavy and light chains have not been determined. It is estimated that each β-sandwich contains one disulfide bridge with additional hydrogen bonds. The shows how the four heavy and light chain β-sandwiches fit together. The Fab region heavy chains attach to the Fc region heavy chains via the Hinge region (blue link), before continuing down into the intracellular domain to interact with the Igα/Igβ subunits. The light chains however are only connected to the heavy chains within the Fab region, thus have no contact with the subsequent domains.

Figure 3. Surface Representation of IgM Antibody Binding Pocket.

Signal Transduction

The diagram in Figure 4 depicts the initial process of B cell activation by the antigen binding to the antibody at the Fab region. The underlying mechanism for signal transduction is unknown but it is speculated to operate under what is known as the conserved assembly mechanism (blue link). This means that upon antigen binding, BCRs on the surface of the cell begin to cluster to cause the phosphorylation of the immunoreceptor tyrosine-based activation motifs located in Igα and Igβ. In its “off” state, the constant region 4 of heavy chain B overlaps the extracellular components of Igα and Igβ. As the antigen binds, it induces a conformational change to release the overlap and allow for clustering about the BCR. Now, in its “on” state the phosphorylation of the ITAM region (blue link) (observed here as the conserved tyrosine residues are phosphorylated) within the intracellular tails of Igα and Igβ drives downstream kinase activity to continue to process of signal cascading (blue link).

Figure 4. IgM Antibody Signal Transduction following Antigen Binding.