Fig. 1.

RS1 is predominantly a 16-mer arranged as back-to-back octamer rings. (A) A silver-stained blue native gel showing an oligomer of ∼400 kDa, much larger than an octamer (triangles indicate the expected band position). (B–E) Selected 2D class averages derived from micrographs of frozen-hydrated RS1 particles: (B) top view average; (C) oblique view average of double octamer rings; (D) side view average of double octamer rings; (E) side view average of single octamer rings. (F–M) Selected slices through the 3D reconstruction. (F) Central slice at the interface between the two octamer rings, the arrow indicating one of the narrow connections between the rings. (G) Slice 16.5 Å from the center, with the two β-sheets indicated by dotted arcs and two β-strands indicated by arrows. (H) Slice 33 Å from the center, the arrow indicates a node in the ring (the site of an intermolecular disulfide bond) around the diffuse density in the middle (8 RS1 domains). (I) Guide to the slice levels in F–H. (J) Central slice on a twofold axis, the dotted arc indicating a diffuse density composed of eight N-terminal RS1 domains. (K) Slice 10 Å from the center showing the node (arrow) corresponding to that in H. (L) Slice 48 Å from the center, showing the two curved β-sheets with low density in between (arrow). The latter is occupied by bulky side chains in the core of the domain not resolved in this map. (M) Guide to the slice levels in J–L. (Scale bars, 50 Å.)