A functional analysis of PCNA-binding peptides derived from protein sequence, interaction screening and rational design

Proliferating cell nuclear antigen ( PCNA) has no intrinsic enzymatic function, but functions as a sliding platform to mediate protein interactions with the DNA strand. Many proteins interact with PCNA through a small conserved motif with consensus QxxLxxFF. This work uses Schizosaccharomyces pombe and human cells to analyse the function of PCNA- binding peptides. Interacting peptides were identified using two- hybrid screening; one ( pep102) binds directly to a physiologically relevant site on PCNA. The EGFP- pep102 overexpression phenotype is consistent with competitive blocking of PCNA - protein interactions. Various PCNA- binding peptides were all shown to inhibit PCNA function by competitive binding in both human and S. pombe cells as EGFP fusion proteins. The action of a p21( WAF1/ Cip1)- derived peptide was complicated by the presence of additional functional domains and possible post- translational modi. cation. The activity of pep102 was hampered by low expression in both model systems. The peptide derived from rational design ( con1) was stable, highly active in inhibiting PCNA function both S. pombe and human cells and showed a high affinity for PCNA both in vitro and in vivo. These results validate the use of functional screening in yeast to identify peptide aptamers that are functional in mammalian cells; such aptamers provide excellent leads for small molecule antiproliferative therapies.