Structural domains comprising this superfamily share the structure of the A' domain of subunit A of the pentameric bacteriophage DNA clamp loader, involved in the loading of sliding clamps onto chromosomal DNA. These clamps are involved in processivity of DNA replication.

Pentameric clamp loader complexes belonging to the AAA+ family of ATPases. The A subunit is comprised of two domains termed A and A', both are involved in bridging the gap in the clamp between the A and E subunits, with the A domain (a vestigial AAA+ module) on the lower part of the clamp and the A' domain at the top. Thus, they enable the interaction with the ATPase site of the E subunit.The A' domain domain is described as a collar domain with a C-terminal extension PMID:22194570.