Figure 3

Trimer–trimer interactions in the bovine E2bCD cubic core. (A) Hydrophobic contacts in the 'knob-and-socket' interactions between two-fold-related subunits (blue and green) from each neighboring trimer in the bovine E2bCD core. The residues in the hydrophobic sockets are colored in orange. (B) The same trimer–trimer interface shown in (A) with the residues forming H-bonds and ion pairs as stick models. (C) Pattern diagrams showing trimer–trimer interactions in the cubic cores of the bovine E2bCD (left), E. coli E2oCD (middle) and A. vinelandii E2pCD (right). The protruding C-terminal helix is labeled with a white letter C. The hydrophobic 'knob-and-socket' interface is colored in orange. White lines represent H-bonds and ion pairs. (D) Sedimentation coefficients of wild-type and mutant bovine E2bCDs. The sedimentation coefficients (s_20,w) of the wild-type (red) and R240C mutant (blue) are 17.1S and 17.3S, respectively, which are close to the calculated coefficient (17.5S) from the theoretical molecular mass of the 24-mer E2bCD cubic core. The sedimentation coefficients of the K252N (green) and C2 (orange) mutants are both 4.4S. When modeled as discrete species, the calculated masses of the mutants agree well with the theoretical mass of a E2bCD trimer.