Introduction

Structure

Antigen Binding Site

Heavy Chain Interactions (Igα and Igβ)

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 . Additionally, the Fc portion binds the 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 Igα/β heterodimer. Furthermore, Hc1 and Igα interact through two hydrogen bonds which are stabilized by sandwiching of aromatic residues. Similarly, Hc1 and Igβ interact through three hydrogen bonds. 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 (reference link Qiang "IgM B cell receptor"). 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 (reference link Pavel and Susan). Igα and Igβ 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 Igα/β interface.

Transmembrane Interactions

Function

Proposed Conformational Changes

Signal Pathway

Medical Relevancy