Forms the sliding-clamp-loader together with the small subunit. The clamp loader holds the clamp in an open conformation and places it onto the DNA. The sliding-clamp-loader consists of 4 large subunits and 1 small subunit. Interacts with the sliding clamp; this interaction allows the sliding-clamp-loader to open the sliding clamp. Part of the replicase complex that includes the DNA polymerase, the polymerase clamp, the clamp loader complex, the single-stranded DNA binding protein, the primase, the helicase and the helicase assembly factor. Belongs to the Tevenvirinae sliding-clamp-loader small subunit family.