clamp loader (RFC) bound to the DNA sliding clamp (PCNA)
Saccharomyces cerevisiae (Baker's yeast)
DNA replication is a process required in all organisms before any of its cells divide into daughter cells. DNA replication involves the accurate duplication of the vast amounts of genetic information encoded in the DNA. It requires the splitting of the double stranded parent DNA into two single strands, unwinding of the parent strands and creation of two new strands using the two parent strands as templates. DNA polymerase is one of the primary enzymes required for DNA replication within a cell. It adds one deoxyribonucleotide at a time in the 5'-3' direction to this pre-existing primer to extend it. However, the DNA polymerase has a tendency to frequently dissociate from the DNA template. This prevents rapid replication over long stretches of DNA. Processive DNA polymerases can perform rapid DNA replication without dissociating from the template strand. This processivity is due to the presence of a ring shaped protein called the sliding clamp, which is placed at newly primed sites by the ATP-dependent clamp loader complexes.
This structure shows the yeast clamp loader complex, RFC, bound to the yeast sliding clamp, PCNA. (Fig.1). The RFC complex is consists of 5 subunits that are positioned on top of the PCNA and are tipped away from it. Each of the 5 subunits contains 3 domains. Two of these domains - an ATPase domain and a small helical domain - form the AAA+ module (AAA+ is a superfamily of ATPases Associated with diverse Activities). This module is connected to a third helical domain, which forms part of the collar of the RFC complex. The RFC complex binds ATP (ATP analog, ATP-gammaS, shown here), such that the 5 ATPase domains (one from each subunit) form a right-handed spiral which corresponds well to the grooves of the DNA double helix. The authors suggest that these domains spiral around the primed DNA double-helix resulting in the clamp loader complex locking onto the DNA.
Protein Data Bank (PDB)
Bowman, G.D. O'Donnell, M. Kuriyan, J.; "Structural Analysis of a Eukaryotic Sliding Clamp-Clamp Loader Complex"; Nature; (2004) 429:724-730. PubMed:15201901
author: Ashwini Patil