(
A) The 60 nm cuboctahedral cage is formed by twofold symmetrical vertices linked by rods. The angle alpha is 60°, the angle beta is 90° (see
Figure 2—figure supplement 2A for definition of alpha and beta) (
Stagg et al., 2006). (
B) In the 90 nm icosidodecahedral cage (
Stagg et al., 2008), alpha is 60°, but beta is 108°, leading to formation of pentagonal faces. (
C) If beta was sufficiently flexible to form 120° angles, hexagonal faces could form, leading to assembly of a flat coat, as hypothesized by
Stagg et al. (2008). Combinations of square, pentagonal and hexagonal faces into the same cage could potentially increase the extent of size variations allowed for the COPII coat.