Solution binding of immunoglobulins to immobilized Igαβ at 540 RU of surface densities. (A) The binding sensorgrams of recombinant Fcμ at 10, 5, 2.5, 1.25 and 0.625 μM concentrations to immobilized Igαβ. The calculated dissociation constant K_D is 2.3 ×10⁻⁶ ± 1.1 ×10⁻⁶ μM. (B) The binding responses of 3μM Fcμ and various human immunoglobulins to immobilized Igαβ. The responses were taken at the time point immediate before the dissociation phase.

Murine and human Igβ homodimer formation. (A) Murine Igβ homodimer with secondary structure elements labeled. Monomers A and B are colored cyan and lemon, respectively. (B) Detailed view of the hydrophobic core and hydrogen bonds (black dashed lines) at the homodimer interface with major side chains involved in the interactions. (C) Human Igβ homodimer. Monomers A and B are colored green and yellow, respectively. (D) Detailed view of major side chains involved in the interactions at human Igβ interface. Cystine residues are shown in stick representation and labeled red. See also Figure S3.

Sequence alignments of Igβs and Igαs. (A) Sequence comparison of several mammalian and chicken Igβs. The numbering is consistent with the sequence of murine Igβ. The secondary structure elements of Igβ are illustrated as arrows for β-strands. Cysteins are highlighted red. Residues involved in interactions at the interface of the murine Igβ homodimer are highlighted according to the subunit color, i.e. cyan and yellow for A- and B-subunit, respectively. Residues involved in the interactions at the interface of human Igβ highlighted green. (B) Sequence alignment of murine and human Igβs and Igαs. The numbering is consistent with murine Igβ (bold font) and Igα (regular font). Cysteins are highlighted red. Unique cystein 98 in murine Igα is highlighted magenta. (C) Sequence alignment of several mammalian Igαs in the area of predicted G-, G’-strands. Conserved residues are highlighted gray. See also Figure S4.

Ribbon drawing of a monomer from murine (A) and human (B) Igβ homodimers, respectively. All secondary structure elements on Igβs are marked in accordance with the sequence alignment in Figure 4. Cysteins (labeled in red) and disulfide bonds are shown in stick representation, N- and C-termini are marked. (C) Structural comparison between murine Igβ (cyan) and human Igβ (yellow) monomers. Positions of disulfide bonds are shown with arrows. Labels colored according to the color of corresponding molecule. (D) Structural comparison between murine Igβ (cyan) and V_L domain of IgG1 (red, PDB code 1YQV). Cysteins and disulfide bonds are shown in stick representation, labeled according to Igβ sequence. C’- and C”-strands, that are different in the two structures are labeled and colored according to the color of corresponding molecule. This figure and all subsequent ribbon drawings are prepared using the program PyMol (DeLano, W.L. The PyMOL Molecular Graphics System (2002), http://www.pymol.org). See also Figure S2.

Binding of mutant human Igα and Igβ to recombinant human Cμ2-Cμ4. (A)Surface representation of human Igαβ heterodimer model. Point mutations resulting in reduction or increase of binding circuled red and green, respectively. Mutations causing absence of protein expression are circled black. Alpha and beta chains of Igαβ heterodimer are colored pale blue and yellow, respectively. (B) 180 degree rotation of the previous view of the Igαβ heterodimer model. (C) Binding of Igα (grey bars) and Igβ (white bars) mutants to Cμ2-Cμ4. Mutant concentration in the analyte was kept constant at 4μM. At least three sets of experiment were performed for each mutant. A table with mutant description is given for reference. (D) Structural model Cμ3-Cμ4 domains of IgM. IgM chains are painted pink and grey with groups of amino acid selected for mutations in red. Groups of mutations μ2 (R485, E486, S487) and μ8 (P562, H563) that affect IgM surface expression are circled. Arginine 485 and proline 562 are highly concerved in IgM. (E) 90 degree rotation of the previous view. (F) Relative surface expression of different IgM groups of mutants (grey bars); white bars represent relative surface expression for IgM YS/VV construct. The YS/VV mutation in transmembrane domains of IgM allows surface IgM expression without binding to the Igαβ heterodimer. That indicates that the effects of the mutations were likely through disrupting interactions with Igαβ. A table with mutant description is given for reference. See also Figure S5.