Crystal Structure of the DNA Polymerase Processivity Factor of T4 Bacteriophage

The protein encoded by gene 45 of T4 bacteriophage (gene 45 protein or gp45), is responsible for tethering the catalytic subunit of T4 DNA Polymerase to DNA during high-speed replication. Also referred to as a sliding DNA clamp, gp45 is similar in its function to the processivity factors of bacterial and eukaryotic DNA polymerases, the beta-clamp and PCNA, respectively. Crystallographic analysis has shown that the beta-clamp and PCNA form highly symmetrical ring-shaped structures through which duplex DNA can be threaded. Gp45 shares no sequence similarity with beta-clamp or PCNA, and sequence comparisons have not been able to establish whether it adopts a similar structure. We have determined the crystal structure of gp45 from T4 bacteriophage at 2.4 A resolution, using multiple isomorphous replacement. The protein forms a trimeric ring-shaped assembly with overall dimensions that are similar to those of the bacterial and eukaryotic processivity factors. Each monomer of gp45 contains two domains that are very similar in chain fold to those of beta-clamp and PCNA. Despite an overall negative charge, the inner surface of the ring is in a region of positive electrostatic potential, consistent with a mechanism in which DNA is threaded through the ring.