Sandbox Reserved 1772: Difference between revisions

While the antigen binding site structure of the mIgM BCR is identical to other common soluble antibodies, there are several intracellular interactions between the heavy chains and Ig''a/B'' subunits that make it unique. In the Fc portion of the structure, the two heavy chains interact via a disulfide bond and form an <scene name='95/952700/O-shaped_ring/1'>O-shaped ring</scene>. Additionally, the Fc portion binds the <scene name='95/952700/O-shaped_ring/2'>Ig''a/B'' heterodimer</scene> with 1:1 stoichiometry (reference link Pavel and Susan). Due to the orientation of the heavy chains in the O-shaped ring, only Heavy chain 1 (Hc1) forms direct interactions with the Ig''a/B'' heterodimer. Furthermore, Hc1 and Ig''a'' interact through two hydrogen bonds which are stabilized by sandwiching of aromatic residues. Similarly, Hc1 and Ig''B'' interact through three hydrogen bonds. The residues involved in the interactions at the heavy chain and Ig''a/B'' interface are highly conserved across all species, suggesting a conserved mode of interaction (reference link Qiang "IgM B cell receptor").  The Ig''a/B'' heterodimer is an obligate component of all BCRs. Ig''a'' and Ig''B'' non-covalently associate with mIgM, and are crucial components for initiating biochemical signaling inside the B cell upon antigen binding (reference link Pavel and Susan). Ig''a'' and Ig''B'' are associated by a disulfide bond between cystine residues. The disulfide bond is stabilized by π-π stacking and a hydrogen bond. These residues are highly conserved across species, suggesting conservation of the Ig''a/B'' interface.