p15PAF Is an Intrinsically Disordered Protein with Nonrandom Structural Preferences at Sites of Interaction with Other Proteins

We present to our knowledge the first structural characterization of the proliferating-cell-nuclear-antigen-associated factor p15PAF, showing that it is monomeric and intrinsically disordered in solution but has nonrandom conformational preferences at sites of protein-protein interactions. p15PAF is a 12 kDa nuclear protein that acts as a regulator of DNA repair during DNA replication. The p15PAF gene is overexpressed in several types of human cancer. The nearly complete NMR backbone assignment of p15PAF allowed us to measure 86 N-HN residual dipolar couplings. Our residual dipolar coupling analysis reveals nonrandom conformational preferences in distinct regions, including the proliferating-cell-nuclear-antigen-interacting protein motif (PIP-box) and the KEN-box (recognized by the ubiquitin ligase that targets p15PAF for degradation). In accordance with these findings, analysis of the 15N R2 relaxation rates shows a relatively reduced mobility for the residues in these regions. The agreement between the experimental small angle x-ray scattering curve of p15PAF and that computed from a statistical coil ensemble corrected for the presence of local secondary structural elements further validates our structural model for p15PAF. The coincidence of these transiently structured regions with protein-protein interaction and posttranslational modification sites suggests a possible role for these structures as molecular recognition elements for p15PAF.