Extended Data Figure 5: Structural comparisons of the dimer-of-dimers model from single particle cryo-EM with crystallographic cis and trans dimers.

a, Crystallographic cis dimers of γB7_EC3–6 (blue ribbon) were aligned with the dimer-of-dimers model (space fill, colors as shown in Fig.1) over the EC5–6 cis-dimer regions derived from γB7_EC3–6 (black bars). The EC4/EC5 linker regions appear to accommodate a high degree of structural variation.

b, Crystallographic γB2_EC1–5 trans dimers (blue ribbon) were aligned with the manually positioned EC1–2:EC3–4 dimer fragments (black bars) in the dimer-of-dimers density. Deviations derive from differences in rotation and bend at the EC2–3 and EC3–4 linker regions within the antiparallel EC1–4 trans dimers.

c, Comparison of the EC4:5 interdomain deflection angles between the dimer-of-dimers model (left) and the crystallographic γB7_EC3–6 cis dimer (right), highlighting the variations between them. Individual EC domains were defined as axes in UCSF chimera and are shown as cylinders. All interdomain deflection angles are listed in Extended Data Table 2.

d, The dimer-of-dimers model was assembled by rigid-body fitting into cryo-ET density of four-domain trans (EC1–2/EC3–4) and cis (EC5–6/EC5–6) units from the γB2_EC1–5 and γB7_EC3–6 crystal structures, respectively. The figure depicts the deflection and rotational angles between these docked units in the final dimer-of-dimers model (left) compared with those in the γB2_EC1–5 trans dimer (right), highlighting the conformational change required within the EC1–4 trans interaction to facilitate formation of the dimer-of-dimers.

e, Deflection and rotational angles between EC5–6/EC5–6 cis-interaction and the EC3–4/EC1–2 trans-interaction units in the repeating unit of the crystallographic γB4_EC1–6 zipper array for comparison to the dimer-of-dimers model.