STRUCTURE OF THE C-TERMINAL REGION OF P21(WAF1 CIP1) COMPLEXED WITH HUMAN PCNA/

The crystal structure of the human DNA polymerase delta processivity f actor PCNA (proliferating cell nuclear antigen) complexed with a 22 re sidue peptide derived from the C-terminus of the cell-cycle checkpoint protein p21(WAF1/CIP1) has been determined at 2.6 Angstrom resolution . p21 binds to PCNA in a 1:1 stoichiometry with an extensive array of interactions that include the formation of a beta sheet with the inter domain connector loop of PCNA. An intact trimeric ring is maintained i n the structure of the p21-PCNA complex, with a central hole available for DNA interaction. The ability of p21 to inhibit the action of PCNA is therefore likely to be due to its masking of elements on PCNA that are required for the binding of other components of the polymerase as sembly.