Proliferating Cell Nuclear Antigen (PCNA) is a homotrimeric DNA sliding clamp involved in DNA replication, repair and cell cycle control. Understanding determinants of mechanical stability of a PCNA monomer composed of two N- and C-terminal domains requires all-atom and coarse-grained steered molecular dynamics simulations. Three schemes of unfolding at a single molecule level indicate that the conserved curvature of the PCNA monomer is due to aggregation of side p-sheets. The force spectra suggest that rupturing an interface between the domains is less probable than opening an interface between the two monomers during PCNA loading onto DNA by Replication Factor C. (C) 2015 Elsevier B.V. All rights reserved.