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 Igα/β 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/9'>O-shaped ring</scene>. Additionally, the Fc portion binds the <scene name='95/952700/O-shaped_ring/7'>Ig α/β heterodimer</scene> <scene name='95/952700/O-shaped_ring/2'>(heterodimer zoomed)</scene> with 1:1 stoichiometry. <Ref name="Tolar P"> Tolar P, Pierce SK. Unveiling the B cell receptor structure. Science. 2022 Aug 19;377(6608):819-820. [doi: 10.1126/science.add8065. Epub 2022 Aug 18. PMID: 35981020.] </Ref>. Due to the orientation of the heavy chains in the O-shaped ring, only Heavy chain 1 (Hc1) forms direct interactions with the Igα/β heterodimer. Furthermore, <scene name='95/952700/Ig-a_and_hc_1/3'>Hc1 and Igα interact</scene> through two hydrogen bonds (T75-Q487 and N73-Q493) which are stabilized by sandwiching of aromatic residues (W76 sandwiched between F358 and F485). Similarly, <scene name='95/952700/Igb_and_hc/1'>Hc1 and Igβ interact</scene> through three hydrogen bonds (Y66-R491, K62-T530, and R55-T533,). The residues involved in the interactions at the heavy chain and Igα/β interface are highly conserved across all species, suggesting a conserved mode of interaction. <Ref name="Su Q"> Su Q, Chen M, Shi Y, Zhang X, Huang G, Huang B, Liu D, Liu Z, Shi Y. Cryo-EM structure of the human IgM B cell receptor. Science. 2022 Aug 19;377(6608):875-880. [doi: 10.1126/science.abo3923. Epub 2022 Aug 18. PMID: 35981043.]</Ref>.  The Igα/β heterodimer is an obligate component of all BCRs. Igα and Igβ non-covalently associate with mIgM, and are crucial components for initiating biochemical signaling inside the B cell upon antigen binding. <Ref name="Tolar P"> Tolar P, Pierce SK. Unveiling the B cell receptor structure. Science. 2022 Aug 19;377(6608):819-820. [doi: 10.1126/science.add8065. Epub 2022 Aug 18. PMID: 35981020.] </Ref>. <scene name='95/952700/Iga_and_igb/1'>Igα and Igβ are associated</scene> by a disulfide bond between cystine residues (C119-C136). The disulfide bond is stabilized by π-π stacking (Y122 and F52) and a hydrogen bond (G120-R51). These residues are highly conserved across species, suggesting conservation of the Igα/β interface.