Figure 4

Structures of clamp loaders from different branches of life and in different bound states. The five subunits of the clamp loader - A, B, C, D and E - are shown in different colors. (a) The structure of the clamp loader of E. coli (known as the γ-complex) in the apo form (PDB code 1JR3) [114]. This structure illustrates the three conserved domains of clamp loader subunits. The two amino-terminal domains constitute the AAA+ module. The carboxy-terminal domains form a disc-like structure that holds the complex together as a tight pentamer. (b) Structure of the budding yeast clamp loader, replication factor-C (RFC), bound to the sliding clamp, PCNA, and an ATP analog (PDB code 1SXJ) [59]. ATP induces a spiral arrangement of the AAA+ modules. The clamp is not open in this structure, probably because of mutations in key interfacial residues (the arginine fingers) that disrupt the tight interactions between adjacent AAA+ modules and prevent hydrolysis of the ATP analog. (c) Structure of the T4 bacteriophage clamp loader bound to primer-template DNA, an open sliding clamp and ATP analog (PDB code 3U60) [60]. The duplex region of the primer-template junction is bound within the central chambers of the clamp loader and the sliding clamp, with the 5' single-stranded template extruded through the gap in the clamp loader where the missing sixth subunit would be. The AAA+ modules of the clamp loader, bridged by the ATP analog, form a spiral that perfectly matches the helical symmetry of DNA. Movies of the 3 structures that form the upper panel are available by clicking on them in the PDF version of this article. They are also available as additional files 1,2 and 3. (Adobe Reader Version 8 or higher required).