Figure 1.

Human PCNA in complex with p21 peptide (1AXC) [2]. PCNA is a trimeric molecule, with each monomer containing a PIP-box binding site and located between the C-terminus of the protein and the interdomain connector loop (IDCL). The human PCNA in complex with p21 shows the classic mode of interaction, with the conserved glutamine of the PIP-box motif interacting both directly and via solvent molecules with the clamp surface. The turn of 3₁₀ helix and the immediately C-terminal region insert the hydrophobic section of the motif into the hydrophobic cleft on the PCNA surface. The extended peptide forms an interaction with the IDCL. Each PCNA monomer is coloured separately with the p21 peptide in magenta. Key features are labelled. Also shown is the PIP-box consensus motif, where a indicates a small hydrophobic residue, h is an aromatic residue, and x is any amino acid [1]. Figures, unless otherwise stated, were produced using PyMol [28].