(
A) A close up view of the vertex in the cuboctahedral cage (
Stagg et al., 2006), with the X-ray model for Sec13/31 dimers fitted into the half-rods. The vertex forms upon interaction of four N-terminal Sec31 β-propellers. The β-propellers on two heterotetramer rods (purple) face each other at the center of the vertex and are referred to as the ‘plus’ ends of the rods, while the β-propellers on the other two rods (green) are further away from the center, and are referred to as the ‘minus’ ends. In the cuboctahedral and icosidodecahedral cage geometries, each rod has a plus and a minus end. When viewing the vertex from outside the cage, alpha is defined as the clockwise angle from ‘+’ to ‘−’, and beta the clockwise angle from ‘−’ to ‘+’. These interactions allow assembly of a coat that curves in two directions, appropriate for coating spherical membranes. (
B) A close up view of the rod segmented from the edge of the cuboctahedral cage. The rod is bent in the middle at the Sec31 dimerisation interface by an angle of 135°. (
C) A surface representation of the atomic structure of the Sec13/31 heterodimer as solved by X-ray crystallography (
Fath et al., 2007), filtered to a resolution of 40 Å. The structure is overall very similar to the cuboctahedral cage edge, but the bend in the middle of the rod is less accentuated, with an angle of 165°.