Claspin is loaded onto DNA replication origins during replication initiation. Studies in Xenopus egg extracts indicate claspin loading requires the presence of Cdc45, a factor that promotes the initial unwinding of the origin DNA in the presence of Cdk2. This step is followed by RPA binding which is a prerequisite for recruitment of PCNA and DNA polymerases alpha and delta. As RPA is not required for claspin binding, it is postulated that claspin binds at the time of initial origin unwinding but prior to the initiation of DNA synthesis. Claspin would then continue to associate with replication fork machinery where it can serve as a checkpoint sensor protein. Even though associated with the replication fork, claspin is not an essential DNA replication factor.
Studies of Xenopus claspin indicate that it can physically associate with cognate Cdc45, DNA polymerase epsilon, RPA, RFC, and Rad17-RFC on chromatin. Studies of purified human claspin indicate that it binds with high affinity to branched (or forked) DNA structures that resemble stalled replication forks. Electron microscopy of these complexes indicates that claspin binds as a ring-like structure near the branch. The protein is hypothesized to encircle the DNA at these sites.