Fig. 5: The β₂ clamp partially or completely closes on loading onto primer-template DNA.

a Cryo-EM map of the clamp loader complex (CLC) bound to a partially open clamp and p/t DNA (CLC•DNA•β₂^open). Densities corresponding to individual subunits are coloured as in Fig. 2. Density of the template DNA strand is coloured gold and the primer stand pink. Grey density at the top right corner belongs to SSB. b Cryo-EM map of the CLC bound to a closed clamp and p/t DNA (CLC•DNA•β₂^closed). c Cryo-EM map of the CLC bound to p/t DNA (CLC•DNA). d The CLC constricts slightly on binding of p/t DNA. The AAA+ modules of CLC•β₂^open (coloured grey) and CLC•DNA•β₂^open are superimposed to show small inward movements of the CLC upon p/t DNA binding. Arrows point to the tips of the δ subunits. e Orthogonal views showing β₂ partially closes on p/t DNA and also becomes spiral. β₂ of CLC•β₂^open in the left pane is coloured grey and CLC•DNA•β₂^open yellow and orange. Most movement occurs in the β-I subunit. β₂ of CLC•DNA•β₂^open becomes lock washer-like (right). f,g CLC−β interfaces on β₂ and CLC of CLC•DNA•β₂^open and CLC•DNA•β₂^closed, respectively, showing that the latter has much reduced contacts. h Mg•ADP•AlF₄⁻ molecules are present in all three ATP-binding sites of CLC•DNA•β₂^closed and the arginine finger residues: R169 of γ2 and γ3 and R158 of δ’, coordinate the AlF₄⁻ groups that mimic the γ-phosphates of ATP. Mg²⁺ (spheres) and AlF₄^– ions are coloured in shades of green and EM densities of Mg•ADP•AlF₄⁻ are shown as mesh. Hydrogen bonds between ADP•AlF₄⁻ and residues of the CLC are shown as red lines.