Figure 2.

Structure of the complex. (A) View from the back of the λ Exo trimer, looking into the channel from which the nascent 3′-overhang would emerge. The dsDNA substrate would enter the channel on the opposite side. The three subunits of λ Exo are colored yellow, cyan, and wheat. The three Redβ CTDs are shown in green, each bound to a single λ Exo subunit at the periphery of the complex. (B) Side view of the complex, from the left of panel A. The paths of the dsDNA substrate and ssDNA product are indicated. Notice that the C-terminal domains are bound on the side of the λ Exo trimer from which the 3′-overhang would emerge, and that the N-terminal domains, which would connect to Thr-195 (labeled for the CTD in front), would be well positioned to bind to the emerging ssDNA. (C) Close-up view of the interactions at the interface, zooming in on the boxed region of panel B from the lower right, as indicated by the arrow. Hydrogen bonds (within 3.5 Å) are shown as dashed lines. (D) View from the left of panel C, to show the interactions behind helix E, although Thr-95 of helix E of λ Exo is still occluded in this view.