Structure of D′D3 assembly. (A-B) Two views of D′D3 showing a rectangular face (A) and triangular face (B) of the D3 wedge with dimensions. (C) Interactions that stabilize the E′-VWD3 junction. (D) View of the face of D3 slightly below which cysteines implicated in D3 dimerization, mutated to alanine here, are buried. Figures prepared with PyMol show ribbon cartoon and disulfides and N-glycans with white carbons in stick, red oxygens, blue nitrogens, and yellow sulfurs. Important residues are shown in stick and labeled. Ca²⁺ is shown as a silver sphere. In panels A and B, the Cβ atoms of the C1099A and C1142A mutants are shown as spheres. In D and subsequent figures, A1099 and A1142 are modeled as cysteines, using the allowed rotameric position of the S atom, as discussed in Results. Dot surfaces around the S atom emphasize that there is room for it in the structure, and that it is a model.