The sliding clamp (also known as theβ-dimer or theβ-clamp) is made of two identical polypeptides that are encoded by dnaN. X-ray crystallography studies reveal that head-to-tail interactions between the two semicircular shaped polypeptide subunits give the sliding clamp subassembly the appearance of a ring. The diameter of the hole within the ring is about 3. 5 nm and therefore big enough for a double-stranded DNA molecule to fit inside.

    Each polypeptide chain has three domains. Even though the amino acid sequences of the segments making up the three domains are different, each domain has the same folding pattern, consisting of two β-strands on the outside and two α-helices on the inside. The net effect is that the β-dimer appears to have a sixfold rotational axis of symmetry even though it only has a true twofold rotational axis of symmetry. Twelve α-helices (2 polypeptides * 3 domains/polypeptide* 2 α-helices/domain) line the inside of the sliding clamp.

    If we make the reasonable assumption that DNA is perpendicular to the plane of the sliding clamp, then the axis of each α-helix will be perpendicular to the major and minor grooves, blocking the protein from gaining access to either DNA groove and allowing the clamp to slide along the long axis of DNA. There is sufficient space for one or two water layers between the DNA and the sliding clamp, suggesting that the clamp may "ice skate" along the double stranded DNA. Twelve α-helices line the center of the sliding clamp. The arrangement of the α-helices in the sliding clamp is in marked contrast to their arrangement in DNA-binding proteins, which usually have their α-helices parallel to the grooves so that the helices can fit into the major groove. The carboxyl ends of the two polypeptides project from the same face of the sliding clamp and serve as points of association for the remainder of the holoenzyme. Thus, the sliding clamp increases the holoenzyme's processivity by acting as a tether for the remainder of the holoenzyme.