Deletion studies to reveal the basis for size discrepancy in proliferating cell nuclear antigen

Proliferating cell nuclear antigen (PCNA), an essential component for DNA replication in eukaryotes, is a highly conserved nonhistone nuclear protein of 261 amino acids. The molecular weight of mammalian PCNA, estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), differs notably from that predicted by the cDNA sequences, that is, 36,000 in comparison with 29,261 and 28,748 for human and rat PCNA, respectively. To investigate if this discrepancy is due to posttranslational modifications, we studied the PCNA protein synthesized by an in vitro transcription/translation system as well as the protein overproduced in bacteria. We found that both PCNA protein samples were indistinguishable from the authentic protein from the protein mobility in SDS-PAGE. The finding indicates that the size discrepancy is not due to the posttranslational modifications. Hence, the size discrepancy may be due to the protein sequence per se, namely a sequence-related anomaly in SDS-PAGE. Results from the analyses of a series of PCNA derivatives with various lengths of C- or N-terminal deletion indicate that the putative sequence is in the region of residues 128-150.