Oligomerization of hIKK2 dimers in the crystal.

<p>(A) Three successive asymmetric units, each composed of six protomers, are taken from the hIKK2 X-ray crystal structure and depicted with the center asymmetric unit (labeled A–F) and colored with surface rendering as in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001581#pbio-1001581-g003" target="_blank">Figure 3A</a>. Within this arrangement can be found four unique but closely related versions of the same dimer of dimers. Beginning from the left, there are four protomers (D, E, and another D′ and E′ from the symmetry-related asymmetric unit) that assemble into a tetramer. These four polypeptide chains are depicted as opaque, while the remaining protomers are rendered as semitransparent. In the second depiction of the same assembly of three asymmetric units, chains B, C, D, and E are rendered opaque and reveal themselves to assemble with a similar tetrameric arrangement. Likewise, protomers A, B, C, and F in the third panel and A, F, and the symmetry-related A″ and F″ chains assemble into renditions of the same tetramer. (B) Close-up views of the four unique assemblies viewed perpendicular to their 2-fold rotation axes reveal their close similarity.