After Igα and Igβ dimerization, the transmembrane helices of the heavy chains can embed within the B-cell membrane and intertwine with the Igα and Igβ chains. ^[4] The side chains of this , made up by the alpha, beta, and heavy chains (A/B), are primarily hydrophobic; this allows for interactions with the hydrophobic tails in the phospholipid bilayer. The four helices (Figure 2) are intertwined and primarily held together through interactions between the ; however, a a few polar residues are included which allow for additional interactions with the polar residues on the Igα and Igβ chains. ^[5]

Figure 2. 4-pass integral helix. PyMOL image of the integral helices in IgM BCR (PDB:7xq8). The structure is shown rotated on the x and y axes to illustrate how the chains are intertwined. 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 chain A and heavy chain B associate (Figure 1) asymmetrically to facilitate intracellular signaling cascades. The allows them to pack together via Van der Waals 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 hydrogen bond, 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 chains within the BCR. ^[2]

Furthermore, both the Igα and Igβ chains have cytoplasmic tails that extend into the B cell (Figure 1). Each of these tails contain an ITAM region to facilitate signal transduction (Figure 4). ^[3] The structures of the ITAM regions have not yet been determined.

Fc Region

The constant region of IgM is made up of the two . These heavy chains form a bridge connecting the FAB region or variable region to the transmembrane region (Figure 1). They also act as a wire that the variable region can send a signal through to the transmembrane region as a mechanical change.

help hold the heavy chains and Igα/Igβ chains together in the extracellular portion of the transmembrane 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 structure as the antigen binds. In constant region two, which is located at the start of the Fc region, heavy chain A and heavy chain 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. ^[3]

Fab Region

The Fab region of the antibody is where antigen recognition occurs upon binding (Figure 1). On each arm is one heavy (A/B) and one light (A/B) chain, both containing domains identical to their respective counterparts. Repeats of β-sandwiches form the constant and variable domains within the Fab region. 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 . ^[9]

The IgM-BCR contains areas referred to as complementary-determining regions(CDRs), which are where the antigen makes 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 (A/B) and light (A/B) chains have not been determined. Based on the general structure of an antibody 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, before continuing down into the intracellular domain to interact with the Igα/Igβ subunits. The light chains (A/B) however are only connected to the heavy chains (A/B) within the Fab region, thus have no contact with the Igα/Igβ heterodimer.

Figure 3. Surface Representation of IgM Antibody Binding Pocket. On one arm of the IgM antibody, the antigen makes contact with light chain A at the L1 and L3 complementary-determining regions. Furthermore, it makes contact with heavy chain A at the H1, H2, and H3 complementary-determining regions. The location of the complementary-determining regions were approximated using the structure of the VCR01 variable region and were visualized using PyMOL.

Signal Transduction

Figure 4. IgM Antibody Signal Transduction following Antigen Binding. At the end of the intracellular Igα and Igβ helices are their cytoplasmic tails, and on each tail are tyrosine residues that are phosphorylated by one of two tyrosine kinase enzymes: Splenic-tyrosine kinase and Src family kinase. While the specific tyrosine residues are unknown in the mechanism, it is understood that their phosphorylation activates the B cell by triggering downstream intracellular signaling.