FIG. 3.

Cryo-EM reconstructions of Ad2 (left) and Ad12 (right) at ∼21-Å resolution. (A) Ad capsids viewed along an icosahedral threefold axis. The penton base proteins at the icosahedral vertices are shown in yellow, the reconstructed portion of the flexible fibers are in green, and the remaining capsid density is in blue. (B) Side views of the external portion of the penton base contoured at a level corresponding to the strong capsid density. (C) Enlargements of a single penton base protrusion at two isosurface levels, one just above noise (transparent red) and the other showing well-defined density (yellow). (D) The lengths of the variable regions flanking the RGD sequence (red) in Ad2 and Ad12 are obtained from sequence alignment of five different Ad serotypes. The scale bars are 100 (A) and 25 (B and C) Å.