Speed and processivity of replicative DNA polymerases can be enhanced via coupling to a sliding clamp. Due to the closed ring shape of the clamp, a clamp loader protein, belonging to the AAA+ class of ATPases, needs to open the ring-shaped clamp before loading it to DNA. Here, we developed real-time fluorescence assays to study the clamp (PCNA) and the clamp loader (RFC) from the mesophilic archaeon Methanosarcina acetivorans. Unexpectedly, we discovered that RFC can assemble a PCNA ring from monomers in solution. A motion-based DNA polymerization assay showed that the PCNA assembled by RFC is functional. This PCNA assembly activity required the ATP-bound conformation of RFC. Our work demonstrates a reverse-chaperoning activity for an AAA+ protein that can act as a template for the assembly of another protein complex.
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